Dave, you're right. You could get a bigger gearratio. Technobots has a Z10 and a Z120 mod 1 gear in store. Combine this with a 125mm wheel and you have a 12-1 gearratio, with 1.5mm groundclearance below the gear.
You also can use the trick of the first RAF machine in RW, the very flat, triangular, torpedo armed (pneumatic spike) , if I recall correctly, Typhoon. They milled the 12mm axle of the Bosch 750 so it became a Z10 gear.
Putting this in a 6mm axle, you get a Z4,

Several reasons why I give a Z12 Mod 1 gear the preference.
Speed 900 has a 6mm axle. Some Johnson variants (the Speed 900 is a Johnson motor) have a 1/4 axle.
A Z10 has a inner diameter -for the teeth of 10-2.5mm= 7.5mm, 1.25mm or 1.1mm of steel under the teeth. Do you think that is enough?

Guideline of mechanical enginering is max gearratio with normal gears or sprockets in 1 stage is 8 to 1. Everything above that is rape.
I did find some 10-1 epicyclic gearboxes from an Italian manufactorer. But those are rated very low on imput force.

Sorry to be conservative in this respect. But I dare to say that the track record of the MSR single stage drivetrains is proven.

I know of some combat failures.

1) Hannibalito I, using an MSR 12-54 @10S NiMH, at the XFM Halloween special, after a hit on the unprotected 900 by Little Hitter, what shattered 1 magnet. (this was solved by adding 5mm of HDPE armor)
2) Kashei, what used the MSR 12-54 with 6S Lifepo4 on 110mm wheels. The low grade ali gears didn't take the strain, and shredded teeth like a maine coon cat in the Sahara.(this was solved by electronic restraints put in the TX, and the new variant will use less V, 6-1 gearatio and 7075T6 ali gears.)
3) Plofbox, after a hit from Hornet straight on the gear.(unprotected wheels.... what a surprise)

4) Satanix is a strange case (that uses another MSR drive). It wasn't dependable. Untill I had the opportunity to work on it. I still don't know what the secret was. The only thing I did was giving everything just a tad more clearance.