The bigger question I have is about the subboard that sits ontop of the Gary and is sodered to the motherboard.
I have one of these in one of my A500
subboard probably for the Ad540
http://amiga.resource.cx/exp/adram540It allow for more than 1 meg of ram to be mapped from the A500 trapdoor slot.
Two things to remember, 1), as you mentioned, the ram above the 2.5 megs needs a command (usually early in the startup-sequence) to mount it.
2), The "fast" ram is not truly fast. It is mounted in the "fast" ram memory space, but is limited to the trapdoor speed.
This is usually referred to as "slow-fast" ram. So that it is not confused with true 16 bit "fast-fast" ram.
The amiga will be faster than an A500 without fast ram, but not quite as fast as an amiga with true fast ram.
ie relative speed: slow ram < 16 bit fast ram (usually zorro II or 86 pin expansion) < zorro III 32 fast ram < 32 bit fast ram on motherboard or processor expansion board
