Disconnecting the solenoid will lower your power - not increase it.
GROUNDING the solenoid will increase your power below 4500rpm (as it will essentially bleed out and remove the 4500rpm solenoid switchover).
At those levels of modification, 0.8mm is also probably overkill - I wouldn't bother, and if you were having issues, I'd probably be thinking fresh coil-packs prior to a smaller plug gap. Unless you're running much higher boost, its almost certainly un-necessary and will also rob you of power.
Realistically, 3" exhaust, FMIC, ground out the solenoid (or even better - EBC), nistune + retune.
Look at things like coil-packs / plugs if you see flat-spots / mis-firing at that point. If you already have flat-spots, mis-fires, you should get that stuff sorted before trying to get more power out. Start from a solid base.