Has this engine had the piston lightened..?
I'm no mechanical engineer, but it seems that the smaller you go in engine size the more smooth [intrinsically] it ought to be at higher revs.
I've heard of tinkerers using out of balance props to diagnose whether the piston is too heavy or too light by doing short trial runs. According to Clarence Lee a steel piston model plane engine is usually going to be heavy on the piston side once you reach a high enough rpm to reveal that.
I took a bunch of weight out of several .049 pistons and felt that was always a win - win to improve smoothness and increase the red line and longevity. With the load carried by the piston dome, you can make the walls as thin as paper..or as thin as you dare.