As already indicated by the contributions above... any increase in horsepower will most effectively improve takeoff/climb performance, and only marginally improve cruise speeds while concurrently increasing fuel consumption..... or can maintain previous cruise speeds at near previous fuel consumption rates.
In other words, like Bruce mentioned, using horsepower to increase cruise speeds is not the most efficient method to accompish that goal. This can be made more evident by recalling the axiom that "DRAG...increases as the SQUARE of velocity!"
Also, ... on 100 mile trips, you'd have to go a WHOLE LOT FASTER to realize any benefits from increased speed. (It's alot cheaper to simply depart 15 minutes earlier.)
The most cost effective method to increase speed in these airplanes is to REDUCE DRAG by removing unnecessary antennas, proper rigging, and quit carrying around unnecessary weight like all the junk which accumulates in the baggage compartment. Plan your fuel more efficiently and quit carrying around 42 gallons of fuel to make that 100 mile flight.
(Carry 20 gallons instead and save 120 pounds of weight. You'll get off the ground shorter, climb faster, and get there faster and not burn as much fuel doing it.)