This is a WiP
I thought I'd have a go at creating a thread where we consolidate all our tips, tricks, and best practices for optimal performance. I'll convert the questions into whatever tips we get.
Mounting
Tips on how to internally mount in various Atari models.
My STacy:
Links to STL files that help mounting or completely encase CosmosEx
Wifi
Are there any settings on RPi or AP that would improve wifi connections?
5GHz networks don't seem to work?
My 5GHz did not.
List of known Wifi dongles
RPi 3 B+ built-in Wifi; anyone have flawless performance?
My STacy has this and Wifi works fine for little things. Shared drive over Wifi is hit-and-miss. FTP through Litchi is better but can be iffy. Not flawless in the least. Well, Litchi is near flawless and a fantastic FTP-client.
Any other Wifi dongle with flawless performance
Flawless meaning fully working shared drive and file transfer over wifi without errors.
SD Card
What brand/model/size works better in stark contrast to others you own?
No clear winner between my cards.
How many partitions is too much in terms of reliability?
Does the boot drive partition size matter, that is, be 32MB or lower?
What driver have you gotten the best results using your best SD card?
I've settled for 2GB Kingston over 8GB SandDisk (Class4), 32GB Kingston, and 1GB no-name with driver as ICD.
Do you have write verify on and does it offer substantial benefits at a cost of performance?
Write verify is on in ICD with visual cues on menubar reporting of write errors.
Did you use to have write errors and how did you solve them?
Better PSU solved most of my errors. Some SD card/driver combinations seem to work better than others. Before switching the PSU, PPDRIVER was definitely more stable than HDDRIVER.
HDD Driver
What driver have you chosen and was it because it worked better than the alternatives?
I own the latest HDDRIVER and PPDRIVER but I've currently settled with ICD. It appears to work better.
Any must-have settings to know? (sector,cache,write verify,partition size/type)
Did you use to have write errors and how did you solve them in the driver?
Power
How are you powering CosmosEx?
Using STacy internal DC/DC supply and 3rd-party 16V 4amp brick.
Have you noticed a difference between different PSUs in terms of performance or reliability?
Most of my instability and reliability issues were fixed by swapping the DC/DC supply with a spare. Both are recapped units. I suspect that my flaky one needs the output voltages adjusted. I actually had the low battery light on the STacy. Swapping external bricks didn't help but certainly is better than the original 2.5amp one. I might experiment with using a quality external PSU direct to CosmosEx to see if it improves anything further.
CosmosEx: Best Practises
CosmosEx: Best Practises
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Re: CosmosEx: Best Practises
So, my spare DC/DC psu is even flakier. Took the previous one to my testbench STacy and measured voltages. 5V was waay off spec. Almost 6V 
So I put some load on the psu and started adjusting voltages. The trimmers are really crap. Too sensitive and probably just installing it could have changed the value. Note to self; always measure voltages before closing the STacy up.
Some observations:
There are three trimmers on the STacy DC/DC board. Two are labeled 12V and 5V. The third one seems to make no difference to voltages. Gotta investigate that.
This DC/DC is not generating a -12V for some reason. I gotta see what it's even used for.
I put the original DC/DC board back and the low battery light was gone and writing to SD was clearly much more stable. I was able to read and verify 5x 5MB files in Jookie's test without errors for the first time.
So in my case having a clean stable and bang-on +5V has made the single biggest difference in SD performance. So much so that there is now also a clear difference between my cards.
The 2GB Kingston is slower at writes than the 8GB SanDisk and that in turn slower than the 32GB Kingston.
I then ran a comparison with the three drivers with Jookie's test.
PPDRIVER: Potentially the fastest writes but ultimately failed and was not able to complete the test. Needs write verify.
HDDRIVER: Almost as fast as PPDRIVER without write verify. Very respectable speed with write verification but ultimately has occasional write errors and verification has issues.
ICD: Slowest writes (but acceptable) of the bunch with write verification on but the only driver to manage to consistently pass write and verification without issues. The visual cues on the menubar do show constant errors but apparently it retries until successful.
So ICD a clear winner in this use case without a doubt. I've spent hours to reach this conclusion and I'm confident this is true in my case. Obviously another setup on another Atari model might be different but I believe any differences are relative to my findings.
So I put some load on the psu and started adjusting voltages. The trimmers are really crap. Too sensitive and probably just installing it could have changed the value. Note to self; always measure voltages before closing the STacy up.
Some observations:
There are three trimmers on the STacy DC/DC board. Two are labeled 12V and 5V. The third one seems to make no difference to voltages. Gotta investigate that.
This DC/DC is not generating a -12V for some reason. I gotta see what it's even used for.
I put the original DC/DC board back and the low battery light was gone and writing to SD was clearly much more stable. I was able to read and verify 5x 5MB files in Jookie's test without errors for the first time.
So in my case having a clean stable and bang-on +5V has made the single biggest difference in SD performance. So much so that there is now also a clear difference between my cards.
The 2GB Kingston is slower at writes than the 8GB SanDisk and that in turn slower than the 32GB Kingston.
I then ran a comparison with the three drivers with Jookie's test.
PPDRIVER: Potentially the fastest writes but ultimately failed and was not able to complete the test. Needs write verify.
HDDRIVER: Almost as fast as PPDRIVER without write verify. Very respectable speed with write verification but ultimately has occasional write errors and verification has issues.
ICD: Slowest writes (but acceptable) of the bunch with write verification on but the only driver to manage to consistently pass write and verification without issues. The visual cues on the menubar do show constant errors but apparently it retries until successful.
So ICD a clear winner in this use case without a doubt. I've spent hours to reach this conclusion and I'm confident this is true in my case. Obviously another setup on another Atari model might be different but I believe any differences are relative to my findings.