Mixture screw works only at slow idle (ralenti setting) so useless playing around. Before sending engine to dealer, swap and test 18 ignition coil on 25 horse, secure well coil body to ground. Both are "swapable" the only difference are spark cables lenghts, longer on 25 than on 18.
Probably that's the culprit, if it's ok, could be a CD related issue, or a faulty head gasket, which I doubt being a horse with low hours use, right ? Spun prop is discarded, you mentioned that engine lacks punch and bogs down at neutral or in gear when accelerating.
Is 380 planes fast with an 18, imagine what a 25/30 will do, will fly you to Mars..