(Sorry for double reply in a sense)
I would be able to test both having things sped up, and FGPA based 1351 mouse support (also 2nd test via MouSTer, but the actual 40 year old tank mouse and old 40 year old hardware I don't have anymore). Very confident in 99% the same test results as old hardware.
Speed and Turbo toggles on the C64U are completely independant from all the software, they can be enabled and disabled at will, but on a C128 it's a register (unused on the C64) but might get overly complicated with any software that updates the graphics this much. It's a timing thing the only safe use of 2MHz is confined to the border/vblank time window. Like how Elite 128 does it. I'm sorry I didn't bring it up upfront, it was way in the back of my head and I know very little, all in all.