Finally I have installed the SDMA clock patch. I decided to go with Exxos patch after Paul testimony and also a discussion with Exxos about the simple resistor patch which should be working too but could represent a risk at long term. (Problem of tension on some components)
So the results are : my Falcon is still working, I can now copy from and to SCSI in VGA 256 colors mode without any corruption, in extended Videl 800x592x256c I can also copy files under TOS and MINT without the error message. So a great improvement.
But, as there is a but, in extended resolutions 256 colors there is still a corruption of files during transfer that I can only detect when doing a verification with unzip of a large archive transferred from SCSI to IDE (unzip -t archive.zip detects a bad CRC)
Also, for whatever reason, Videl Inside is no longer working after the patch, only Centscreen. It's not a big deal, but it made me sweat a bit.
The good thing with Centscreen is that you can deactivate the screen (Ctrl +shift+alt), and therefore free up the bus load. In extended resolution 256 colors, with the screen deactivated, there is no file corruption during file copy between SCSI and IDE.
In conclusion, the SDMA patch solves SCSI transfer issue in VGA 256c, improves transfer in extended resolution, but the corruption problem remains under heavy bus load. The work around is to decrease bus load during transfer, thanks to Centscreen screensaver mode, a kind of Macgyver solution
So I believe that it's time for me to go and play with the CTPCI and the Radeon which has just arrived today