Heres a quick video I did this weekend comparing the 3 batteries I use to show what kinds of speeds they'll each reach. Results below, skip to 1:50 to get past the boring stuff!
7 Cell NiMH 3000mah: 45.3mph (better than what I was expecting)
2s LiPo 5800 mah: 47.1mph (not much more for top speed but way better acceleration)
3s LiPo 4000 mah: 66.2mph (just watch the video