The rpm's are higher at 6000 than at sea level, the air is thinner and there is less load on the prop. I live at 1400' and my engines will run faster at home than at the field at 400'. But yeah, the YS kicks @$$.
JJ, I wouldn't think of adding weight without it first being a last resort like, my servos were all next to the firewall. Get 'em out of the tail if your CG is too aft! Engine weight is always an issue unless you absolutely cannot balance without lead. A G62 is going in my Lanier Cap which usually balances with this engine fine. However, mine is modified and will be about 13 lbs with an OS 160. I might try it just to see what fun-fly wing loading is like on this bird!