Does PERFORMANCE POWERBOND 25% UNDERDRIVE KIT WITH PULLEY FORD BA BF FG V8 improves horsepower of vehicle or just improves bottom end take off?

It improves both. The way an underdrive balancer works is that it reduces the amount of power going from the engine to drive engine accessories such as alternator, water pump, power steering pump etc. It also reduces the mass moment of inertia of the balancer meaning that it takes less power to accelerate the engine.