I can’t say I see a difference between 8GB and 16GB on my G5’s (DC 2.3GHz and Quad 2.5GHz). It’s a knowing that it is “maxed out” that makes it seem better.
I use PS CS3 under Leopard and it appears to be limited to 2GB of memory. One way to boost this is to setup a RAM disk (which can only address a maximum of 2GB itself) as the primary scratch disk and then the HDD/SSD as secondary, technically giving PS 4GB of RAM to work with.