As someone who put in a small engine (135 HP O-290-D2) I have chuckle whenever I ready these threads.
First off, there is simply no replacement for displacement. Bigger is better, yatta yatta yatta.
Here is something no one has mentioned yet, weight. From what I've found on the
web, the O-235 is 240 lbs, O-290 is 263 lbs, O-320 is 272 lbs, the O-360 is 282 lbs, IO-360 is 320 lbs.
Depending on the prop, you can add an bunch of weight, or not. I have heard a constant speed prop can come in at 80 lbs by the time you finish with the controller, plumbing, cable, etc. On the other side, my Catto prop is all of 9 lbs.
Why am I saying all this? Because weight is the enemy! Build a light plane and it will climb close to the same rate as an overweight plane. (1,800 FPM for my -9 w/ full fuel and two aboard.) That weight also decreases your fuel efficiency.
The extra weight also cuts into your useful load. My -9 came in at 990 lbs empty with a useful load of 760 lbs. I just can't put that much fuel, people, and stuff in the thing.
Regarding climb performance, how much climb performance do you "need"? Sure going up at 2100 FPM is great but even an O-235 power RV-9 goes up faster than a 172 and cruises faster too.
The truth is, not many people pull back the throttle to save gas when taking a trip. Sure my plane is slower but fuel stop to fuel stop, I have found that the other RV's are just clearing the pumps when I roll up.
Another truth is, I wonder what 160 HP in a light -9 would feel like.
Oh, and a light plane flies better than a heavy one. Just remember, build the plane you want, not the plane others want you to build.