My bride has just gone from 165 to 155 on her TT and reports a massive difference. Seems to engage all leg muscle groups now, stops her 'toeing' and keeps her knees straight up and over. Someone also mentioned, but it has opened up her hip angle nicely & gets her into a much more aero position.
I am about to go from 172.5 to 170 myself, purely to help the longevity of my knees.
Have a look around, but there are articles on lab tests done that show there was no reduction in power output through different crank lengths. For me the longer you can be comfortable on the bike, the bet