Ive been following some threads on a different forum where a member is data logging a number of different ESC/Motor combinations in the same vehicle. He has found that while 20C (assuming it IS a TRUE 20C... which many are not) is more than enough for general use in systems pulling around 100amps, high current systems CAN pull up to 160/170 amps.

If the system is attempting to pull 160 amps and you have batteries that are rated to supply only 100 amps its easy to see how the actual mathematics of it say this is not enough and both the drive and the batteries will be noticing negative effects. In a situation where you system is pulling 160 amps you WILL notice a difference moving to a battery that can supply it.

I own a stampede 4X4 and was running an MMP/2400 SCT combo in it on 20C 5000mah Venom 2S packs. It was fine... performed well. But, I moved to a 30C 6000mah 2S SPC pack (who's C rating I trust implicitly to be true) and I noticed quite a big difference in how the truck felt. Same gearing, same tires, same everything... just a different pack. Both were essentially brand new packs. I went back and forth between the 2 for about 10 days before i finally just got rid of the 20C packs.

Now in my stampede 2X4 ive done the same comparison. Run a 20C and a 30C pack, back to back, back and forth and I cant tell a difference at all, other than on the initial bottom end punch and its so marginal its not worth talking about. The traxxas VXL system is known to pull around 100amps in a 2X4 truck, the MMP/2400 SCT system is known to pull upwards of 160amp (data logged, not estimated) so you can see why im noticing the difference in one and not the other.