Did you mount the elevator servos at the tail? If you add 2 servos at the tail, 1.5 oz each, that's 3 oz at the tail, more than enough to offset 6 oz of extra weight at the nose. If you use high performance servos that weighs 2 oz each, you will offset 8 oz of engine weight, which is the difference between Saito 100 (20.8 oz) & Saito 120S (28.86 oz).