No. You can run a gun off a 20C or 90C battery and it'll give you VERY close to the same ROF. Difference comes from less internal resistance on the battery, not from any magic factors.
It's possible to have a CA piston last years as much as it is a G&P gearset. Fact is CA makes shitty pistons on average.
Go with the 7.4v LiPo, it's the safer option, and you don't NEED 11.1v to raise your ROF, there's more than enough torque available from the battery to push a high speed gearset.
And an airsoft gun does NOT draw 40A unless there's something seriously wrong with it.
A fully upgraded performance gun shooting 400fps at 1200rpm won't draw more than 18A at the very most. Most stock guns on average draw 4-12A. You can test with LiPos, if your gun shoots slower on a 15C than it does on a 20C, it's because there isn't enough amperage being supplied to the motor.
The startup current is what you're referring too, and it's not measurable on a linear scale, because the motor is technically closing the circuit 100% before the armature starts moving. It's over such a short period of time that it really doesn't affect the battery at all.