If you have a 90LX, you already have pipe timing. If you didn’t get the pipe from Dub, then you might be able squeeze a little more out of it depending on the brand of pipe and manifold. Where you can make a little more power is lightening the prop load and getting more rpm going. If you were doing 16k on an 11x7, I’d take the 11x7 and cut it down to 10.5” diameter. I don’t know if APC did a 10.5x7, but anywhere you can reduce diameter a little while maintaining pitch will usually eek a little more out of it.. Maybe go 10x8 or 10x9, or a 9x10. My buddy suggested a pylon prop - maybe 8.8x8 or 9.5x7 on my 56LX.

Lowering the head and/or adding nitro can make a difference, too. It takes a little fiddling to get it just right.