One more thing that you may want to consider with running 2000mAhr packs, and then running 3800's for your races is WEIGHT.
I have quite a few GP3300 packs, and when I got back into Off-road, I bought a two 4200's. I had the same idea as you, run the 3300's in qualifying, and then the 4200's in the finals, but I found it hard to get my truck balanced when switching from the 3300's to the 4200's (for the finals).
I was aware of the extra punch from the high voltage 4200's, but it wasnt until I put the packs on a set of scales that I realized that there is also a 60 gram difference in weight.
Because of this, I have decided to only use 4200's, and I've been doing a lot better.