filegroups in a database can be used for a few things (filestream, memory optimised tables etc etc) but splitting your storage is your main gain.. (perhaps a tiny improvement if you are on separate spindles due to seek times)
I really wouldn't bother splitting, grant is correct - plus it will make your life a misery when you come to do scripting for restores
files in tempdb …. I stick with cpu count -1 (up to 8) mainly due to reading brent ozar's articles about locks and latches...
we fixed a lot of stuff by doing that, but tempdb is different to any other db, it is used by a lot more things (unindexed tables, bad joins etc)