So with your re-written firmware, what does this wee beastie actually achieve in terms of real bandwidth, etc. etc.
125 MSPS sampling rate. The real bandwidth is unknown, but it is high enough for the sampling rate.
IOW is it worth me buying one installing your firmware?
Only if you want a "hackable" device you can modify for your needs and generally mess around. If you just plan to use the firmware as is, it is probably too early. There are some features missing that I would like to implement, but I need to take a break for some other projects. I'll get back to it eventually.
Overall, the analog performance is mediocre, especially when it comes to compensation. My personal preference would be to redesign the whole PCB, and I have no issues with the digital part, but the analog part would require some experimentation, and I don't have time for that right now.