I doubt there will be any slowdown. We all know how much of a perfectionist KY is, and he likely has a standing rule that any employee or tester who sees a problem, no matter how small (like slowdown) is to report to him immediately. He'll then go about figuring out the exact cause of the slowdown, and either repair it or remove it (if it's something like an extra tree or whatever).
Slowdown is caused by a very simple problem, that's the same regardless of the game: The software is trying to get the hardware to do something it just can't do fast enough. Basically, the system is being pushed too far, and a certain effect or object is just too much for the system to render at full frame-rate. This can happen for several reasons, but I can think of only two "top" reasons why: 1) The game was rushed through development for the sole purpose of money (holiday season, rushing a sequel to bank off the success of the first game, etc). Or 2) The developers want to achieve a very specific look to an object/texture/effect, and are willing to take a small hit to frame-rate in order to make it work.
An example of #1 would be the new Price of Persia.. which has gotten decidedly negative reviews from users for being rushed, and apparently has a very unstable frame-rate. An example of #2 would be Killzone. The game looks fabulous, I don't think anyone can argue that.. but they had to make sacrifices in order to make it look as good as it does. Maybe a few too many sacrifices, according to some players, but that's a discussion for another place.
Now, GT4 doesn't fall into either of those two categories. I think we can all agree that the game hasn't been rushed... And I think we all know KY well enough to say that he's not going to make any sacrifices in order to make the game pretty. Especially with that little gizmo they have that tells them how the system is handling under what they're doing to it. That tells them exactly where they need to back off on something, or where they have room to add something more, without taking it over the limit. With the constant fine-tuning the game's been under for the last couple years, I think it's safe to say we're going to get a game that not only pushes the system to it's limits, but does so in an efficient manner without going over those limits, like Killzone did.
Oh, and like a couple of the others, I never noticed any slowdown in GT3... if it was there, it was so small I never saw it. And again, this time they have that gizmo.. they didn't have that when they made GT3. (I really need to find out what that thing's called, dammit...)