Regarding speeding up disc accesses: what I did on model-b was have a mode that turned off the speed limiter and only rendered every 50th frame when the disc was being accessed or when a PC key was pressed. This is a bit of a quick and dirty hack, but it worked nicely, and the hacky bit was fairly localized (since keyboard and disc sections just exposed a flag each, which the main loop checked when regulating the speed).
For b2, I did it "properly", by which I mean I did something awful: the 1770 and the 6502 communicate, so that every time the 1770 raises an NMI, instead of waiting some period before issuing the next one, it waits for the instruction after the next RTI, then raises another straight away. The 6502 then gets a stream of NMIs, as fast as it can handle. This mode also removes most disc-related delays (except for the delay between sectors, due to the way Watford DDFS works
In principle this can be foiled, but it seems to be reliable with the DFSs I've tried it with: Acorn 1770 DFS, Watford DDFS, Opus DDOS and Opus CHALLENGER. The code for this was a fair bit more invasive though...