You don't really need to go very fast to have a competitive car in autocross. Here the important thing is really being able to accelerate from 0-45mph (if that). You are right to run as large a tire as you can since the skidpad and slalom are better performance indicators for that type of driving. What I was alluding to on smaller front tires refers more to road racing where speeds can get up into the 60-100mph range. Here you can fudge a bit and (theoretically) run a narrower front tire. In fact you can tune the understeer/oversteer characteristics by playing around with tire size.
But for autocross I'd run as wide a tire as I could all the way around. I'd also suggest getting the Enkei RPF01 wheel as it's the lightest for the money.
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