Sometimes you'll see an engine leaning out in flight because the air is rushing into the carb at an increased rate due to the speed of the plane compared to when it's just sitting on the ground. So, as a general rule, it's best to set the high end slightly rich to compensate for this phenonmenon. You might try richening the high end slightly and see if the performance improves.

But you said if anything you think the engine might be rich already, so the problem might have something to do with your prop selection. Even though the 13 x 6 has worked well on other models, you might want to try a different prop on the new plane. Different airframes react differently to motor/prop combinations, and you may find a different pitch or size on your prop will give you the perfomance you expect.

This is all assuming that the new Sukhoi isn't noticably heavier than the other planes you've had this engine mounted on. If it is, that, of course, will affect how much "punch" it has.

David