You can rebuild packages if you like later (if you do 'emerge -p <package>' it will tell you that the package is present and it will rebuild it then you can continue on), I don't know how easy (or useful) it would be to do a mass recompile (commands like 'emerge world' would only check for upgrades and I don't know of any way to force recompilation on a large scale).
It's unlikely that you will really notice a speed increase of any sort, mostly you can benefit from things like USE flags where you can sayand get a package emerged for use with gnome and gtk2 but without kde and arts support -- this does depend on the package having those options in the first place of course ... you'd have trouble emerging kde with gnome and gtk2 support .Code:USE="+gnome +gtk2 -kde -arts" emerge something
Good luck with whatever you choose.