As stated by others here and elsewhere. Run a gallon of fuel through it before trying for all it's got, or forming an opinion of your new engine. With proper break-in, just about any engine will bring you years of joy. And don't rule out your original ST90. A gal of low nitro, 18-20% oil fuel will show what a great motor these are. As for your desire of a 4stroke, same advice. A gal of fuel before an honest opinion can be made (or more on some of my bigger 4 strokes).
-Snuts-