If your system is stable enough to not need a reboot between launches, then you might have a drastic improvement by jumping that much RAM.
It's more about what else causes things to page out in the mean time than how photoshop behaves. In practice with Lion and Snow Leopard - it seems that the difference between 4 and 8 G of RAM makes a noticeable speedup after several days use have the programs you use resident in RAM. Going to 16 would be heaven to me. Even 5 seconds is pretty fast, so don't get it if you are going to be disappointed if it's 4 or 5 after the upgrade.
Other things will sped up for sure, but it's hard to know if your workload will guarantee 2 seconds with any amount of RAM if the inactive pages that hold Photoshop eventually get released by the virtual memory system to make room for newer inactive pages.