Admittedly, you will be down a little on power by going with a 2-cell LiPoly instead of an 8-cell NiCd or NiMH. But, it's not as bad as it might seem on the surface. Here's why:

The NOMINAL (rated) voltage is 9.6V for an 8-cell NiMH, and 7.4V for a 2-cell LiPoly. Fully charged, the NiMH will read at least 10.4V (up to 11.2V), and the LiPoly will read 8.4V. Even worse, right? Hold on a second, we haven't fired up the motor yet!

With the motor running, you'll get what's called voltage depression on both batteries. Under full load, a properly sized 8-cell NiMH will drop to about 8.8V, while a properly sized 2-cell LiPoly pack will drop to about 8.0V. That's not a huge difference, less than 10%.