@Gray "Performance" is much like "strength" in that when it comes to details there are various ways to break down these terms and could mean different things.
To make an analogy the "performance" of a car depends on the "requirements". A F1 car is great on a track made for an F1 car but isn't going to "perform" the task of dragging a payload up a mountain for that case you're better off with a semi truck or something with more torque but lower top speed.
In the world of quads and specifically to your question about battery cell count or voltage the trade off is you are making it heavier but you are giving the motors a higher max RPM. The motors are rated in terms of "KV" this term essentially tells you how many RPM per volt applied, 1000kv does 1000rpm at 1V and does 2000rpm at 2V (when duty is at 100% or it is sending the full 2V it will get up to 2000RPM). The extra speed on the prop will create extra up force but the weight of the extra cells will add more down force so there is some balancing point. The heavier the quad is the more inertia it has as well so the less agile it becomes and more likely it is to run into wobbles that it doesn't have the control authority to balance out.
To show some more real world numbers too a 2600kv motor with no load (no prop or anything slowing it down) at 2S would have a max RPM of:
4.2 (max charged voltage) x 2S x 2600kv = 21,840
4.2 (max charged voltage) x 4S x 2600kv = 43,680
That said this is "no load speed" with a prop on there the force that it feels from the air is related to the surface area that hits the air and the velocity (squared I believe) so higher velocity on the prop also means more drag it's feeling so won't really get up to either of those speeds and on the higher top speed number will be a more substantial drop since velocity has such an impact on drag force.
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TLDR use ecalc.ch and or give it a shot so long as all the components you feed VBAT directly into can handle the voltage then nothing *should* smoke (YMMV).
