The answer really is "it depends". The light twin I used to fly ( 2x PT6) was certified up to FL310. We used to regularly fly it at FL270, (we were non-RVSM) and it sipped fuel. If we wanted to get there a bit faster, we had a better TAS at FL210-FL240. A decent head / tail wind would remove any of those savings.
The large twin turboprop (PT126's) I also had time on was the same, but about 10,000' lower. Horses for courses.