Somewhere (I can't find the thread now) Larry Renger (IIRC) recommended drilling out the Cox production engine's venturi, which narrows down beyond the NVA, to the existing exterior diameter. I tried it with one of mine, and it sounds more like a Black Widow than a Babe Bee! And still starts, draws fuel, and needles just fine. So my thanks to Larry, and you guys oughtta try it if you want to make your Surestarts into superior sport engines.
No official tach comparisons, sorry. But a significant upgrade according to my ear.