Smokey...the stock tank is 11 ounces, and yes you can fly 10 minutes without alot of wide open throttle. Today I flew for 11min 30 seconds on the stock tank mostly at half to three quarters throttle, but did use full throttle for about 8 full power passes/uplines. On this flight I was mostly flying trimming routines and then the IMAC basic sequence twice, along with 4 touch and goes.

I use the rather large stock muffler that came with the OS91FX and mounted it as all the way back against the mount bolts. The batteries (4-cell NiMh) is mounted behind the fuslage spar immediately behind the battery tray, and I used a wooden 15x6 zinger prop (weighed 1 oz. less than APC). The back of the tank is flush with the rear of the tank compartment ( to reduce the amount of cg shift from full to empty tank). NO WEIGHTS was added to get it to the recommended cg-- only placement of battery and using light wooden prop.
Tomorrow will I will probably move the battery to the front side of the spar to move the cg a little forwared for a tad more locked-in feel, for I intend to use this plane for IMAC basic practice.