I couldn't imagine just that giving an extra 1-2 horses, but I've never used them personally so I cant say. The idea seems like it's there. Same concept as a golf ball. I believe Mercedes even used dimpled matting on the bottom of some of their cars to reduce wind noise and make the car slip through the air better.
I don't know whether or not it did anything, but when Toby at MotoXperts did my head he dimpled the back side of my intake valves. He said he has seen just that make small gains. I doubt 2 hp, but every little bit helps and it was only 50 bucks for both of them.
i believe dan from dasa said he tryed them in a 250f motor or something and called it a waste of money....i would probably stick to a regular good set of valves and put the money somewhere else where its proven