hey there,
I have the same setup you do. Here's a few facts for comparison.

I am flying the global ultimate ARF with Saito 91. I run the MA 14x6 wide chord prop. I use 20%nitro with 20%oil (all synth). I'm using the OS F plug. I am flying at less than 500ft elevation in 80% humidity and near 90degrees F. This engine peaks at 10,200. I run it a tad rich at 9600rpm. Its still very new but even at this rpm it climbs forever vertically. I weighed this thing too. Ready to fly weight with fuel in the tank was an astoundingly heavy 7.8lbs. I'm using a 4 cell NIMH pack and only 4 servos. Granted some are hi torq BB servos, but I did not add additional servos. I added 1.5 ounces to the tail for my liking. It will fly without any but, I like a more pitch sensitive model.

Things to consider.
What is your elevation and temp?....higher elevation will make difference

What fuel are you using?

If they are the same or better perhaps you simply have a poor performer out of the box..??

Incidently, I bought two Saito 91s a the same time. The other is installed in a Hobbico Extra 300 60size. It also came out to the same weight ready to fly. it has the same power to weight performance and runs the same RPMs.

I have 1997 model Saito 91S on a Stinger 60 I've been flying for years. Its a heavy 8.5 lbs and the engine is a little tired but turns a 14x6 9400rpm. It will hold this model at a hover but will not climb out. (again at 500ft and lower altitude).

Hope this helps...

hawk