I noticed the exact same thing, and at this point I've decided that it's nothing more than the extra weight from add-on's causing shorter flight times. I'm averaging about 8 minutes per pack on the stock batteries. I run superskids, titanium superboom, GWS DD tail, G90, aluminum swash & 1 pair of extra flybar weights so I know I'm weighing the bird down, probably enough to warrant a brushless main motor upgrade. One thing I did find that helped a great deal with the extra load was increasing the pitch control arm length by 1/2 turn. The little extra aggressiveness in your pitch curve will help with the added weight. Just keep in mind that it will also affect how your bird handles in idle-up.

Shafts have never been a problem for me, I've replaced 1. Probably because I stocked 5 extras in my part's drawer. Seems I never break the parts I keep multiple extras of! It's the feathering shafts that are my bane, the stock ones bend for me like they're made out of pewter. FlightTech stainless steel feathering shafts here I come... $16 a piece tho, ouch! Honestly the whole stock head on this bird makes me a teeny bit nervous. I've torn it down enough to see all the areas it needs improvement and am actually considering upgrading to a precision machined CNC aluminum head. The only thing stopping me is the $138 the head costs vs. $138 that needs to go into my TREX.