AFAIK It's proper Fast Ram, probably not as Fast as the ACA's but I will make the SysInfo screens later (after FA Cup final)
Jens did mention in a post locations you can Map memory that wont conflict with the ACA500 so would need to double check and see if that could be more then 2mb.
Just bare in mind the cost of buying these expansions vs the cost of buying an ACA12xx, at a point you're better off just buying an ACA1220!