Actually, F1 can run fast with high T motor, dont need much power. The car is very light...around 1.1 kg.
I use 17.5 but use low gearing 2.5 - 3.0 and with turbo, speed not bad.
Probably 10.5T will be better....
In general, F1 speed comes from good (careful) car setup and cornering technique.
X1 may be ok, may need to be creative in how to insert battery.
(where did you buy X1 already? is it available?)
With older 104Pro, either buy option upper deck (like Zen) or be creative with DIY modification.
I used to fit 6000 60C with some modification, but like other C-Hing says, there is no need for high capacity. I now use 3000 - 4000, and to keep weight low as well...