After reading your post I thought to myself "This sounds like the problems the last owner of my ship was having when I bought it". Yes, I bought a fixer-up-er. So sit back and pop open a cold one and enjoy my long story, it might help, it might not, but it might give a place to start.
When I got mine, the last log entery AFTER carb and both fuel pumps overhauled and all 6 cylinders topped (at about 250 SMOH) was that it still was not developing full power and fuel pressure fluctuating at high power settings. Now this sounds much like you. Have you noticed your fuel pressure bouncing around while pulling into a hover? The previous owner got tired of spending all this $$$ and let it sit for a couple years before selling. When I got it home I started trouble shooting starting with the simple stuff. I had one dead mag, pulled both and found dead mag caused by bad points. Replaced points and condensor in both, CAREFULLY set E-GAP, and reinstalled on engine.
Now for the good part for you... You mentioned that you had the mags overhauled recently, so we'll assume they are in good shape. I started checking the fuel system from tank to carb and this is what I recommend you to do.
1. Check the age of all flexible hoses in the fuel system (for that matter ALL flexible hoses). Old hoses can internally crack causing fuel leaks, or swell causing unseen obstructions to flow. Also check that all connections are tight, but not overtight.
2. With the fuel strainer by the electric pump empty, turn on the fuel pump and note how long it takes fuel to start flowing into the strainer and how well it flows in. Does it take a while to start filling? As it is filling does it look like turning on a hose, or just kinda dribbling in?
3. Check fuel pressure after the strainer and carb bowl are full. With the engine not running it should be steady at 9 psi. I set mine at about 11 psi because if the engine is running it will tend to pull the pressure down a bit and I like to see 8-9psi with pump on and engine at high power.
This is where I found my problem. I took more than 3 mins to fill the strainer and carb bowl!! The pump had just been overhauled so I figured it was not the problem. Things to check...There are two check valves located near the electric pump, check that they are not sticking open or leaking by removing and blowing through them against the flow arrow. You should not be able to. Then blow in the direction of the flow arrow to check that they open and again check that they close. Next, one at a time remove each soilt tube line in the system and check for damaged/corroded flairs or nipples. If any found, replace them (that was part of my problem) as this could cause an air leak. Next, check the condition of the rubber seal at the top of the strainer. Again, if this is leaking it will cause the pumps to suck air, not fuel. And lastly, check each drain cock; the one that drains the strainer, carb bowl drain and the one on the top of the strainer. I found the one on the top of the strainer was not sealing causing another air leak. The drain cocks can be rebuild by replacing the little o-ring in the bottom of them. If you replace one, go ahead and do all three.
After doing all this to mine,here is what I found:
I had one piece of tube between the tee on the outlet of the elect fuel pump and the check valve was missing about 75% of the flair. Fabricated new tube and installed. Rubber gasket on top of strainer was leaking, replaced. Top drain cock was leaking, rebuilt all drain cocks.
After doing this, the fuel system would fillup in less than 30 sec and I had a steady pressure, with engine running. And by gosh I had lots of power!!
Here is what I think was going on and un-noticed because I do not have any EGT gauge. The leaks I found in my fuel system were all leaks on the "suction" side of the fuel system, so they did not show up as wet spots or fluid leaks, but instead allowed air to be sucked into the fuel system, displacing fuel. At idle or warmup power, the carb was still getting enough fuel to run happily, but as power was increased the leaky fuel system was unable to keep up and allowed the fuel level in the carb bowl to drop which leaned out the engine lean of peak, loosing power. This would have been seen if an EGT was installed.
Oh, and I wouldn't mess with trying a diffy comp check on it. If compression checks OK dynamically it wouldn't really yeild any good info and you could damage the accessory drive gear doing it (if thats how you would decide to hold the engine) or yourself as you try to hold 80 psi in that cylinder bore (80 psi X 4.5" bore = enough force to break your arm...at least!!).
Hope this helps!! Good luck!!