This was the problem encountered by TCM when they came up with the idea of increasing output of this engine. Their research led them to the GO-300 series engines in which the engine was allowed to increase hp by increasing operating rpm. The problem with that idea was that propellers lose efficiency at higher rpms so in order to avoid the unacceptable loss of prop efficiency ( and poor fuel mileage) they had to reduce the prop rpm. The only way to do that was to go to a constant speed prop. But that increased costs beyond what the market would bear for this level airplane.
So they used a propeller reduction-gear-box on the nose of the engine. (This is like running your car/truck in second-gear all the time. More power to the wheels but horrible fuel economy and increased wear on the engine.) This reduced the TBO to 1200 (from an already difficult to attain 1800) hours. The gearbox also introduced some new failure-modes. And some operating technique difficulties for inexperienced pilots.
So next they tried to improve the fuel economy by using a constant speed prop on the GO-300-E engine. Someone then realized they'd just done what they were trying to avoid.... put a constant speed prop on a 172/175. A simpler solution already existed.... a 182. And it outperformed the geared engined airplane too!
Back to the prop question that started this thread. It's the pitch of a prop that allows the rpm to be developed that produces hp for takeoff... but if much pitch-reduction is undertaken, then the thing will overspeed in cruise (losing efficiency, increasing fuel burn, and losing speed in the process.) If too much pitch is put into the prop, then insufficient rpm/hp is developed to meet acceptable takeoff distances. (And insufficient rpm may be had for acceptable climb in hot/high conditions.)
This is why the type certificate data for this airplane has, listed under it's approved propellers, limitation on the static rpm. (I.E.- Not over 2330, not under 2230 RPM.) Such limitation prevents overspeed of the engine in level cruise, while also insuring adequate HP to meet the takeoff distance data published for the airplane.
If an engine has the ability to drive the standard prop at higher RPM then (and only then) is it likely that more HP has been developed by the engine. The only way to determine that is to have an accurate method of measuring RPM, and making a test run under standard atmospheric conditions ..hence my question about the static rpm.
Exhaust "back pressures" are overrated in it's affect on hp. If an exhaust system provides adequate volume in relation to intake volume, then there is no need to be concerned. (Remember, the reason hot air takes up more volume is because the molecules are farther apart from each other. Such distance between molecules also mandates that the molecules will have less impact upon any exhaust components on their way outta-there... so it's not necessary to have any more volume than intake.) The PowerFlow sytem has a great sales/marketing team. They actually have some folks convinced that their "tuning" helps suck exhaust out of that engine. (I challenge anyone to install a manifold pressure/vacuum gauge plumbed into that exhaust anyplace at all and find a vacuum exists! ) I'd be more concerned about the drag of that huge exhaust pipe they sell and it's affect on my speed than I'd be about backpressures of the standard exhaust system. Just inspect that exhaust system thoroughly each annual. Drag goes up as the square of velocity. Ask the big-tires guys here about that. Wanna increase cruise speeds and fuel economy? Get rid of drag! Get rid of those antennas you don't need... and any radios you don't use. (Got an ADF, but never really shoot NDB approaches with it because you have a GPS? Get rid of that dead weight and it's draggy antenna.) Got brake lines hangin' out in the breeze? Get them tucked in behind that landing gear. (A good reason to have your brakes mounted on the aft side of the axle.) Airplane properly rigged? Or are you flying with rudder or aileron applied to lift a heavy wing?
Most airplane improvements are fun to accomplish and are legal.
