I read somewhere in these forums that developers are only allotted 32mb of RAM per app, anyway, so it shouldn't be an issue on any of these devices.
Furthermore, don't you thinknthe extra RAM in the iPhone 4 might have something to do with HD video recording? Caching 30 fps at 720p probably needs that, yes? In my mind, the extra RAM is most likely not used in the normal operations of any of these devices, and when I upped my 3GS to iOS 4 it was snappy as could be and felt like a new phone. The GPU on my iPhone 4 is probably picking up the rear of the slack in addition to the A4 chip making the new iPhone just that little bit faster.
Awesom link to those bench scores on the first page of the thread, btw. Read it and weep, as it seems to tell the whole tale, doesn't it?