The voltage rating will have no effect, just make the cap physically bigger. the 16v is chosen as its the best value for 2S-3S, as modern 1/10th esc's are rated for. 25v is overkill, and will just result in a bigger cap.
Cheaper ESC's have radial electrolytic caps, wheres the solid aluminium caps produce similar capacitance, but in a smaller package, and are more expensive.
You are better with an array of smaller value (330uf-470uf) solid caps as these will charge faster than the single larger cap, and have a better smoothing effect.
The 2 cap or 4 cap units from hobbywing or HPI are very good. same units as speedpassion and orion, just cheaper.
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