Thanks to all for your comments and feedback. I'm still buzzing a week later!

Based on what you guys have said, I've got all the parts on order to redo pretty much all the electrics for 4WD. So much for prioritising, eh? Hopefully shouldn't be too bad now that I've had a bit of experience. Praying that my measurements are right and it all fits, because it's going to be a tight squeeze.

Sam is correct, BDR runs off a 2x12. Based on what Theo said, that may explain the total lack of pushing power last weekend if the ESC overcurrent protection was constantly kicking in.

If you want tank steering and two motors a side, these are OK and not too pricey. We have one running the weapon motor in Ensign Crusher and I believe Chris has one driving each pair of motors on Luna-Tic.
Cheers for that, I've ordered 2. I've also got a Flysky FS-I6X and 2 new motors. In order to fit everything in, I'm dropping back down to 100mm wheels from 120. This means I've ordered new mounts from Nat with the center to base height reduced by 10mm. The smaller wheels should mean more torque too.

Should be a big improvement, if all goes well!