The difference between the store demos and the final Japanese version is night and day. Hell, the difference between the store demo and the downloadable demo is pretty big.
I haven't noticed ANY slow down. The game actually feels much faster. I honestly don't care about online/offline multiplayer. When the game comes out in the US I will buy it again. A patch to add online multiplayer isn't impossible, they did it for Kameo on the 360, but i'm gonna get the US version anyway. There's nothing good coming out between now and March, so it's definitely worth it to hold me over.
I only played for about 30 minutes but so far i'm very impressed. I would have liked a "Free run" or "Time Trial" mode where I can just mess around on the track alone. That's about the only thing bad worth mentioning at this point. Other than that, this game has lived up to the hype for me.
I find myself often pausing the game so I could circle around and stare at my vehicle. The mud also looks amazing and deforms in real time. Each vehicle feels different. You'll need to take a different path on each course depending on what you're in/on.
Anyway, i'm tired as hell tonight but i'm definitely gonna get into this game every second I get. I already can't wait to get home tomorrow so I can play again