Install base makes hardly any difference when you get to a certain point.
Gran Turismo Prologue sold 385,000 units in Europe first week.
The PS3 install base at the time was 11.4M.
So on a similar install base, GT5:P's European first weeks sales outsold Forza 2's worldwide first week sales.
I'm not questioning that GT is the bigger franchise and that it will outsell Forza - I mentioned a few counter facts to put those you listed in perspective. Yours made it seem like "wow! GT is 10x the franchise forza is!" when the actuality of the matter is that forza is a series gaining fans and momentum, and while GT will outsell it, it won't be completely obscured by GT's shadow.