Here are my thoughts.
most 55 engines will give you 500-800 more rpm on the same prop over the 46.
The strongest 45-46 size engine out there with 9" props is the supertiger 45. Run it stock with a Black Jett muffler on a 9x8 and be over 17,000.
A 10 inch pitch prop on these engines with a high drag plane will not be the fastest. you will find that props in the 8" pitch range will be the fastest.
For the OS 55AX and a "red" jett muffler the APC 11x8 will produce good speed and thrust run a 11x7 for a bit more RPM.
The 55AX on a "Blue" Jett muffler you can run the 11x8-11x9 prop or a 12x7-12x8 prop
The 46 AXII on red Jett muffler the 9x8 -10x7 is the best prop.
The 55AX on the stock porting doesnt like to rev much over 15,000.
Also break in of the engine is very important. the MFG directions are good but not the best for max power.