I wouldn't expect any significant HP gains.
After playing with pulley ratios for some time, I'll relate my experiences. On my '67 (factory A/C and PS so I have a 3 sheave crank pulley) I found that with the stock pulleys if I revved it out a few times I would need to replace the bearings in the alternator. After calculating the pulley ratios I found that at 5000 crankhsaft RPM, the alt was spinning up to some ridiculous RPM. My notes from then have been lost somewhere so I don't remember exact numbers. I looked through the junk stuff at the shop and found an alt pulley that was as I recall about 1/2" larger which solved the problem nicely with no other noticeable effects. A few years later, a rebuilt alternator came through the shop that had a deep groove pulley that was larger yet. I put it on my car after which I noticed that after starting it I would have to bring it up to 1200 RPM or so before it started charging and it would discharge at idle if there were a lot of accessories on. No noticeable power increase. A few years later I was given an aftermarket set of aluminum crank and water pump pulleys. These slowed down all the accessories a little bit and slowed down the alternator a lot. I couldn't run the deep groove pulley any more due to lack of charging so I changed back to the second alt pulley above and have been happy since.
A deep groove pulley acts to keep the alternator to a reasonable RPM but also acts as a guide to keep the belt on the pulley at high RPM's.
