I doubt that swithching brands of .70 4 strokes will help. Mine has the OS and no lead. So adding a lighter engine will require adding lead. Unless you want to get real creative with receiver/battery placement. My vertical pretty much sucks too, 13x6 prop. I would go larger in diameter but I only have about 1" ground clearance as it is. I think a lot of it has to do with the THICK airfoil. I am almost certain that this creates a lot of unwanted drag. It does keep the plane stable at slow speeds but slow is all this plane seems to be. Don't get me wrong, it is a blast to fly this plane, but others are much more capable. My Cap, with the same motor, is an entirely different beast. Verticle preformance is sweet. My ultimate will do a torque roll, but it descends during the maneuver. Looks scale while it does this but there is no hope of it pulling vertical out of the maneuver.

Good Luck, Phil