They're not otherwise aerodynamically identical.
The maximum possible performance from increased horsepower, with *every* other factor being equal, is:
New speed = Old speed * (New horsepower/old horsepower)^(1/3)
Now, where there is horsepower in the equation, it should actually be thrust. So, if you go from an inefficient prop to a more efficient prop you could beat the cube root rule. But you're not going to make a Warrior go 10-12 knots faster with an engine upgrade.
The other thing to look at is total trip speed and efficiency. The higher horsepower *will* greatly improve your climb rate, and if you cruise at a higher altitude where the actual power being developed is the same, you'll be able to get there faster and spend more time there, but it's still going to be a very marginal increase overall.
But, like Dr. Bruce says: There's no substitute for horsepower.
I could agree with you there. The one time I flew a Tiger, at 2400 RPM and 3000 feet it got about 115 KTAS. I wasn't too impressed. I was told by the Grumman aficionados here, though, that 2400 was too low of a power setting.
