The SF is a plain bushing engine so 12k is probably a decent number to shoot for with the 8 pitch prop.

The 91 four strokes usually seem pretty happy turning around 10k. The 11x11 is a pretty light load for the engine but it is on the recommended list in the manual. APC makes a 11x12 which is also on the recommended list.

<< EDIT >> I use a 12x6 on a 61 two stroke. The "load" is similar to a 11x8. My engine is an ASP, ball bearing model, and it turns the 12x6 around 13k but I do not remember the exact number