How will they know Forza 3 is more technically accurate? To reach enough people it will have to earn that name for itself. By the time it does that will Forza 3 itself still benefit from it, or will that slowly build-up over multiple games.? If that the case GT5 can still steal it's thunder.
You forget SB's biggest flaw in the argument - switching from GT5 to Forza 3 is a lot more complicated than switching from PES to FIFA, simply because they are both exclusives.
Anyway, I'm one of those petrolheads and I know a lot of GT diehards, and I don't see many signs of people wanting to make that switch. Almost all of them would not only have to buy a new wheel for instance along with that 360, where they can keep using their Momo, Driving Force Pro/GT, G25 or whatever they've been using so far on PS2/3 and PC.
Forza 2 was a bigger opportunity, because at that time GT5 full was so far off, though even there loyalty to GT was high and only people who were really into the paint customisation switched (temporarily I might add - almost all of them got a PS3 and Prologue).
The biggest chance Forza 3 has is if GT5 is delayed significantly again and the PS3 doesn't get a price-cut. The amount of cars on track alone now in GT5 Prologue is something most of the people I know aren't ready to give up again. They've accepted 6 for long enough, and Race Driver's full grid and tweakable/competitive AI has been a bigger draw for GT owners than Forza 2 ever was.
All this regarding the hardcore of course - and I am one of the most hardcore, because I am one of the very few among those who got an Xbox1 and Xbox360 among others to be able to try out online racing, Forza, PGR, and other racing games supporting Live.
Mind you, on less hardcore level, people who don't buy a system specifically for a racing game, Forza 3 may well be more important, and this is probably a more significant number. And Turn10/Microsoft are definitely working hard to compete with GT - the Le Mans event that was mentioned above, is something that Gran Turismo has been doing for six years or longer - they started well before GT4 even included the Le Mans track in fact, doing endurance races on other tracks before that with big prizes to win. Close cooperation with Logitech for making wheels was present since GT3. Multi-display support using up to six PS2s for creating a full 360 view with up to six TVs was present in GT3 already. Photomode in GT4. All these things Microsoft/Turn10 is picking up on now, which shows you they're still looking to GT for inspiration, and are still looking hard to compete with GT as much as possible.
And I definitely don't mind! Competition is great, and I can always play both. But my guess is it will be a while yet before GT isn't king, especially since pretty much all the complaints of things missing in GT4 are addressed in GT5 to most people's satisfaction.