Of course there is a chance of Forza 3 outselling GT5 but I really doubt Forza 3 can impact GT5 sales by a huge deal if it released with a slimline console, more price drops or whatever.
FIFA did very well last year compared to this year and this year Fifa had several 3 minute adverts on Sky Sports, released earlier and was the better game. FIFA was not doing badly at all previously against the declining quality of PES but forza has not even come close to a single GT game.
Also you are making some serious assumptions, one big one being that after 2 years Sony will not drop the price of the console and secondly the slimline 360 will drop the price by an amount which will seem significant.
There was a bundle for LBP and one for Killzone 2 coming, one for GT5 will also come so Sony will find ways to make sure GT does very well but this is just one of those games which will make people purchase a PS3. Go have a look at the numbers buddy, the franchise is pretty darn strong. ;)
