Your Sportster .20 sounds about the same as an LA .25, maybe a bit less, likely because of the size. We have to use 10% nitro and a MA 8-6 for racing them. Mine does about 15,500 rpm like that, and is about average. A BB FX or AX .25 is a few thousand better, and heavier too. I don't seem to have a Sportster for some reason.