[othello]

First thing i would try at your place:
If your electric components do well on 15/46 and you see some headroom temperature wise, try 16/46. With good batteries (flightpower for sure are) you will see an increase in top speed and acceleration won't really be any worse. But check your temperatures.

A friend of mine runs his Mugen MBX5T like you do with a 1515/1y and MGM 16018, but he uses 6s2p A123 which can be compared to 5s lipo. This translates to good top speed and more acceleration then the car/tires can handle. But the motor does reach 70°C at the end of a 7-9 minute run with 25 degrees outside temperature. Still on the safe side but this tells me, that 6s Lipo would simply be too much -> too much power and every component would run at its limit.

With 5s your voltage increases by 25%. With a good 5s battery your top speed could increase by 15-20% (when you do not change gearing) as your new 5s battery will most likely be heavier then the one you are using now, air drag increases ....

You will definitively use more current, as going faster and getting to higher top speed requires more energy. So runtime will decrease when you choose a flightpower 5s 5000mah (compared to your 4s 5000mah) without changing gearing.