Biggest gearratio you can get in 1 stage on a speed 900 is Z10 mod 1 on the motor and the wheelsize gives you the maximum size of gear.
A Z50 is 52mm external diameter. Giving you a minimum wheel diameter of 55mm.
Chaging to a Z100 and a 110mm wheel won't change speed.
But to give you an idea. Kashei uses 6S and a 6-1 gearratio to a 90mm wheel. It's bloody fast, and uses amps like no tomorrow. We needed to put in a Sidewinder, no other small controler like the Scorpion XXl Ilyama version or the IBC could do anything with it. ( 31km/h is idioticaly fast)
The TZ85's will handle it, but those , in our opinion, lack the fine control the dedicated twin motor controlers give.