I guess bigger fields - as seen in Vision GT game - has very little to do with an offline game.
If 15+ cars appear in the game, it would only have sence in higher-classs races and longer races including endurances. But.
But, I guess 15+ field shown on Vision GT is a teaser for online mode.
No matter that PS3 will be mega-powerfull console, lots of the shear power will be used to add enormous amounts of elements that will make GT5 a true next-gen game. Graphics, sounds, collision-model, upgraded AI behaviour, destructible enviroments, enourmuosley detailed race-atmospherilia, changeable weather conditions, day-night modes, physics model and so on - all of that will be using lots of Cell's power and no matter that many of us - HardCore players - would give anything to have 15+ field - it just won't be possible on one PS3 console - without seriousely loosing some of that details.
But it will be possible on 15+ PS3 consoles.
From the very begginings of PS3 talks, main issue was DISTRIBUTIVE COMPUTING. Distributive capabilities of the Cell will allow taht all elements listed above can persist, together with 15+ cars on the track at once - in online (multi-LAN) mode, where distributive computing is a reality.
Vision GT was an teaser. Of course, one can see it on the way he likes. The slogan "From partial reality to full reality" can be viewed as an hint of what will be going on in off-line, but if it is seen in wider-scale, then it becomes truly alive in online. Where the real racing is. Real-people in real-racing. Full reality.
My predictions goes to no more then 10+ cars in the game on a track in off-line, but of course, I would so love to be positively wrong on that particular issue

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