I'm pretty sure they suck more than just 8 hp up. The Merziere pumps are very reliable and hold at least 3000 hours, for a weekend warrior fun car a lot of time... Benefit is also the proper idle cooling, they will pump a lot more at idle compared to a belt driven pump. If you have already one, I would use it, except your alternator is an old design low AMP.
Whatever amount of hp load on the alternator would need to be taken off the gained figure. I was surprised how much electric fans changed my idle when I hooked them up. On a driver that would be one more part that could fail and leave you stranded.