You know, Duane, that's a great question. I've always gone with at least 400, and usually 600, in the past (both on model planes and on a couple of car resprays!!) and have never had an issue. But I think it's best to check with the manufacturer recommendation on the paint in question.
And, frankly, one of the big drivers on 600 grit was the Dupli-Color high build primer (which I saw on your build log -- I love this primer -- thanks!) I just found that I was moving way too much material with 400 grit for a final sanding... the Dupli-Color is really easy to sand off, and I found it too easy to make an accidental low spot. 400 is perfect for the first pass right after you spray it on, though. So, for the finishing touch, I just moved up to 600 and found that the level of control I had there was much more comfortable.
By the way, although I probably should have checked this first, here's a guide I just found for PPG Deltron, an OEM-style car paint which I'm looking at for the Toni as well. It calls for preparation with 400- to 600-grit paper: