I don't have a P25, but I can share with you the experience I had with my A55. Mine originally had a 60 gigabyte, 5,400 RPM drive in it. The 5,400 RPM drive also had a 2MB drive buffer. I installed a Toshiba unit in place of the factory drive. The new unit was a 60 gigabyte, 7,200 RPM drive with an 8MB drive buffer. The difference was very noticable.
Unfortunately, I don't have raw benchmark numbers. I'd say the additional drive buffer probably made as much difference as the extra 1,800RPM did. You might check to see how big the buffer is on your existing hard drive, and be sure to get a replacement drive with an 8MB buffer. I can tell you that it didn't make much difference heatwise--both drives would make the right palm rest very warm. The difference in battery life was negligible. Under standard usage (most of what I do is word processing to type my notes in class), I lost about 15 minutes of battery life.
I am planning on making a similar upgrade to my P105 (to go from the 100 gigabyte, 5,400RPM drive to a 7,200RPM drive). I will benchmark the system before and after the upgrade this time. I'll be sure to post the results here, in case you haven't upgraded your own drive by that time. Good luck with your upgrade!