basically I think it boils down to higher pitch smaller diameter = more speed, more rpm. more diameter less pitch == better verticle but less top speed and lower rpm which also facilitates longevity in an engine!

I dont know much about the 91 , but 9,000 isnt to bad, .. try the 14 x 8 or perhaps even a 14 X 7 but from what you say about going a bit to fast and needing more verticle the 15 X 6 might be the way to go