Just a note on the 3200 Carbon Lipos they will last roughly 3 times the lifespan as all three of your 4200 NiMH packs combined.
Is their a minimum wieght on the car? If not then you will have an advantage if you set your car up properly.
Also you will not have to worry about charging your batteries so they top out just before your races as every other driver in the know will to gain the utmost advantage by doing this with NiMH. THis is not an issue with LiPo.
As far as hitting the wall the 3200 Lipo will take more abuse from this then will the Stick pack as the stick pack carries with it more weight which means more momentum wich means a harder hit. A couple good wacks and a stick pack can smash the end battery bar through the heat shrink on the battery and create a short. If you short a 4200...well let's just say it is entertaining for everyone not close by.
But I am biased I would check out any of the LiPo threads and see what other users have found.