We run on an indoor carpet track here in Toronto in the winter and we have a mixture of MO3's, 03L's and MO4M's. I am personally using a MO4M and find that it out corners a MO3 and is quicker on short straights. But on the long straight the MO3's have a slight speed advantage. Last summer, outdoors on asphalt the MO3's were the better car but there were only a couple of MO4M's. I will be using one outside this summer to see how it works.
We use these chassis' as a spec class in our club. We allow only ball bearings and HPI X-pattern's as hop-ups. You can also use S-grip treaded tires but other than that no other upgrades. This class has proven to be the most fun.