My experience is to the contrary:

With CSSU, mild overclock to 850MHz and Swappolube I seem to be getting at least as good battery life - possibly better.

I do understand it is hard to be objective about battery life, but I am still on my original, now 12-month-old battery - which must've lost some capacity over that time.

I don't recall any mentions of battery optimisation work included in the CSSU program to date, so I can only conclude that CSSU allows you to get more done in less time.

With 2-3 calls a day, about 2 hrs browsing, approx 25Mb of downloads and playing music/podcasts over FM Tx for approx 1.5 hrs in the same day I'll easily get through the day and often don't need to recharge until 1000hrs the following day, whereas I always used to have to recharge every night.

As a benchmark, my N8 will add a half day to my N900's duration with the same use pattern. The N8 is a contemporary device with what is regarded as highly enviable battery life, so I am very impressed with my 2009-designed N900 with CSSU.