This is something used on some racing circuits and even there it is sometimes difficult to implement, ie used for short periods of time. Can't be used for "highway use".
Problem: mechanical pumps are far more powerful at driving GPM than electric pumps
Very likely you would run into cooling problems as you would not be able to drive the amount of GPM your engine needs.....especially with load, ie off-roading or highway.
Also, the HP gains would be relatively minimal, 4-8 at best likely. Also, account for the fact your alternator will have increased load which also loads the engine (decreased HP) and the longevity of the alternator.
You could also consider and "underdrive", ie smaller pulley (March makes these). This would drive the impeller less which means less work for the engine which means some extra horses. Again, problem is it may not supply the engine with enough cooling.
I like the idea of loosing the cooling fan and replacing with electric fan much better.