Why not simply mount a small tank at what RC-Fiend thinks is the right position (height-wise) but on the outswide of the fuselage temporarily and see if this makes any difference in flight? Not to worry about drag, etc. with a small tank mounted externally as this is done all the time with combat planes using engines of this size. Simply use strapping tape to hold the tank on.

After all, part of the fun of this hobby is to use innovatiion to solve problems.