Hi
In using just Workbench, what speed difference do you notice when:
a) Adding fast memory
b) a 14mhz 68000 plus fast memory
Also is there much difference between a 68020 and 68030 when just using Workbench and loading games.
A) As Paul said, adding Fast ram to a stock A500 provides a modest speed boost, say 5%. The big gain comes from running program code in FAST (CPU only address space) and preserving the CHIP memory that is shared between all of the custom processors. You can run larger and more sophisticated programs with FAST ram added. If you add enough, some of it can even be earmarked as a RAD: (a re-bootable electronic floppy!).
B) There were several 14 Mhz 68000 16-bit accelerators made (like ICD's model) and even a home brew project. I tried both. They were VERY disappointing in performance and compatibility (maybe another 10% when they worked, certainly NOT 2x the speed) due to inadequate buffering and timing collisions. Don't waste your money on one of these!
The 16-bit alternatives were:
1) swapping in a 68010 for the 68000. This was very compatible with all hardware and probably 99.5% of software. Performance claims ranged from 5% to 10%. The last time I did this, the cost was about $10.

This was the only accereration that would not destroy an A570!
2) Supra's Turbo 28 was very compatible and well designed (plenty of buffering and no timing issues). IIRC, performance was over 3 times a stock A500 when coupled with some FAST ram (like Supra's 500RX). This is better than a stock A1200. Sadly, Supra never combined their units, so, you can end up with a number of modules hanging off the side of an A500. I currently have one in an A2000. It's wonderful.

I have less experience with full 32-bit accelerators (68020 and 68030) installed in A500s. I would be concerned about compatibility of other peripherals from mixed manufacturers. Even with these, the addition of some FAST ram (now must be 32-bits wide) greatly improves their performance.