Kernel compiling probably won't be drastically worse on the SSD as just copying the kernel source onto it a few times. Remember that the various small writes are not subsequent writes to the same files, but just the creation of a lot of new files. In this regard, it is really not much different making all those small writes as making one great big write.
In order to build the kernel in RAM would take a very significant amount of RAM. Probably on the order of 8+ GB, maybe 16+. You have tons of RAM? The most obvious way to accomplish this would be to union mount a ramdisk to the kernel source/build path.