Since it runs better with the .8mm gap, it does point to an ignition issue with the larger gapping. But there is more to look at.
The poor performance above 3800 RPM maybe ignition, fuel, or air intake related. It could be the coil failing under load, but it may also be the fuel delivery system not able to supply enough fuel. It may be restricted air flow into the engine.
I'd look at the easy/cheap stuff first. When is the last time you replaced your fuel filter? How about the air filter? See, easy and cheap. :-)
I just replaced my wife's 89 300e coil with an Accel Super coil. Ran much better. See my post regarding that. So if the fuel filter and air filter have been recently changed, the coil would be my next stop. Simple R&R proceedure.
If you still have problems, new cap/rotor..... then fuel pump.
Hope this helps. You may want to see what the "professional" have to say.