Done some quick tests with and with out power savings mode, in short:
Battery life up 20 % better ( really depends on use and settings like screen brightness and so on)
performance impact , well in reality it seems to scale down cpu/ gpu power buy 60 %, but in real life terms the effect is far from that drastic. Most apps run as fin, no real lag increase, games work just fine, even emulator er ok. You do notice a bit of a lag on certain High rez games , with max quality. But so far Still very playable.
FPSE (PSX emu)
Need for speed
VLC ( movie app)
Again these are somewhat subjective tests, and lag is somewhat a subjective element. I haven't done benchmarks and such, simply because benchmark rez don't always translate to real world experience