Until you start setting up planes that require multiple servos on every surface there is no need to even consider a power expander. Even most of the 35% plane do not require their use.

As in anything else, higher quality and performance comes at a higher price. Often the increase in performance also comes with an increased demand for electrical current. The 50cc Sukhoi and your Spektrum radio would be well served with a single 5,200 mAh li-on and regulator from Fromeco or Troy Built. For all intents and purposes they are the same product. I would also suggest a 2,600 mAh li-on but recent readings about low voltage radio shut downs and/or resets makes me leery of doing so.