Compiles on tmpfs are faster, factor is 1.8 to 2 in my tests, provided the working set nearly fits into RAM. With lots of swapping going on, you may end up taking longer to compile. Contrary to what is stated above, tmpfs is not smart about swapping, the data in swap is accessed very randomly and I'd be very surprised if inode data wouldn't also end up in swap on high memory pressure. I found all of this out a long time ago on gentoo trying to compile openoffice with 1G of RAM and a dynamic swapfile manager. Now, with 16G, it is actually feasible.
Another thing: I thought the plan was to migrate to per-user-tmp anyway, somewhere in $HOME, for apps that use a lot of tmp like DVD rippers this would be a good idea anyway.