At 280lbs your probably at a good spot… I have basically the same setup (DMP 70cc kit, 15mm bing) but with 19x38 gearing, “restricted” estoril and 175lbs myself I am doing low 40s and top out at 45mph down a incline. The 19mm carb might make it a bit faster but will mostly make more noise. It would pair nicely with a reed kit setup though.