At higher altitudes you can use higher nitro - when I moved up here to Colorado, I moved up 5% in my nitro to help get back a little bit of the power in my 46FX. I run 30% in all of my saitos. You have lost about 20% of your power at 6000', just like me...

Also, I lost about 800 RPM with the same prop when I moved here, and that was with using 5% higher nitro. So for you, I would say try a lower pitch prop, like a 12X7, if the RPM doesn't go too high, or try a 12x6 three blade prop. (I have used 3-blade props alot, with great success. I don't think they are as inefficient as everyone states, when you compare the same brands.)

Hope this helps - NJD