You've got the transfer port figured out. Don't need much, a thin "wafer" just thick enough to allow the gun to go back together. Never really measured the thickness of one, but by eyeball it looks to be about 3mm thick (and thinking metric isn't natural for me). A couple of inches of tubing would make more seals than you could ever use (as they never seem to wear out).
For the other mods, remember that CLEAN is the key word. ANY stray grit, dirt, metal shavings in the system is going to sooner or later get blown through the valve, and can destroy the valve seal as it passes through.
Polishing means jut that..do not want to alter the size of the tube or barrel fitting in the breech, just want to remove the burrs and any sharp edges that can cut your seals.
The bolt probe can be a problem. It increases vel. by allowing more gas to pass through and reach the pellet. SOME of the bolts have been so hard that they can break when attempts to drill them out are made. The little step-down were the o-ring sits is the thinnest part and it can be snapped off there if you get ham-handed. I personally will just open that port in the side of the bolt probe up very slightly, and open the hollow of the bolt probe a bit.
Shimming the mainspring increases the hammer's force, and as you guess, increases the effort to cock the gun. Doing the smoothing mods, poly port, and opening the bolt probe a little bit increases the gas flow...smacking the valve harder makes the valve open longer, which increases the gas flow (but not all of it is useful gas...if oepn too long, the gas doesn't do much for driving the pellet, it just makes noise). Unless you really want to get the power up, I'd leave the mainspring alone, at least at first.
You're also right about the piercing pin's ability to self center...it's not as good at that once you reduce the size of the head. It's not all that good at self-centering as it is issued for that matter; it's not uncommon to find them rubbing the end of the valve body. It does improve gas flow.
Depending on how fast you want the gun to shoot (and remember, speed costs gas...expect fewer shots per fill). To get the rifle running at a nice 610-635 fps (in .22), seems all you need to do is the polish/deburr, the poly port, and to clean up the bolt probe's oepnings. Might need a hammer shim (a thin one) or might not...the spring strength seem to vary from gun to gun.
