How much space do I need for %TMP% and %TEMP%
As with all things, "it depends." Most of the time you can get away with a few dozen to hundred MBs. However, I've noticed that most .MSI installers get copied to
%TEMP% before Windows Installer really starts to do anything, presumably to make sure that they're local. (And not on a network where latency, bandwidth, disconnections can be a problem.) Sometimes very large logs get created temporarily, that sort of thing. It's easy to end up needing several GB of space for a short period of time, but normal use seems to be < 100 MB on my systems.
Does it matter that it is wiped each time I reboot?
No. In fact, I have scripts I deploy to systems to delete the temp folders of every user and the system-wide temp on startup. Too many poorly written applications simply don't clean up after themselves, or files don't get deleted when a program crashes, and I've seen systems where programs start to fail in unusual ways because there's so many files in the temp folder that it's impossible (or very time consuming) to even find a usable name.
Will it perform better than the SSD in my case?
Sure. RAM is going to be roughly an order of magnitude faster than your SSD, both in IOPS and throughput. Will it be noticeable? I highly doubt it. It might be benchmarkable, but not noticeable. Can the OS do a better job of figuring out what to put in that RAM rather than you attempting to dedicate it to %TEMP%? Almost certainly.