There's always been plenty of low-hanging fruit in the Amiga driver even without a cycle-perfect 68k, and it's great to see them being addressed
Pretty much all graphics effects are handled by the copper, so cycle perfect 68k has never been much of a problem for the amiga. Demos tend to be more affected by kickstart version and instruction caches.
The kickstart version affects demos that 1. make assumptions about the contents of agnus/denise/paula/cia registers. 2. make assumptions about ram usage. 3. make assumptions based on observed behaviour about calls, rather than the documentation.
I've seen claims that some code makes direct calls into kickstart without going through the jump tables, I'm not sure there are many cases of this. It seems to be around the 1.2 time, but there were a couple of 1.2 releases & I don't know how much moved. Most software incompatible with the A1200 can be fixed by softloading kickstart 1.3 into ram, which can't help with this.
