Don't know if it does on Forza because I haven't installed it, but it does speed up most games I've tried. On some it makes a hell of a difference. Fallout 3, Oblivion and Fable 2 were siginificantly improved loading-wise.
Oh and it stops the excessive noise you can sometimes get with the CD drive!
Also, after a few days with Forza 3, it's pretty awesome! Not touched my 360 in months until this came out. Only issue is no Nissan GTR and that really annoying bloke with the really annoying voice.