Since gpu acceleration support on NITDroid is currently basic, it would be very foolish to use that as a yardstick for performance. A much better guage would be it's performance on the 2.2 Motorola Droid/Milestone as they use the same SOC as the n900.

Overclocking in NITDroid is possible, but instructions are not given becuase it's a good way to wreck your device if you don't know what you are doing.