A "power up bulb" is the floating valve (in some pistols it's a rocket-like shape) that "switches" the flow of gas from down the barrel (i.e. to shoot the bb out) to trapped in the nozzle (i.e. to start the recoil cycle).
Illusion would be able to comment in more precise detail, but here is my Layman's understanding.
A heavier bulb will not move forward to switch to the recoil cycle as fast as a lighter bulb/valve. So essentially more gas is shot down the barrel vs. before. More gas typically means more velocity. However, I am of the mind that to really pickup the benefits of that increased amount of gas, a long barrel and heavier BBs are necessary to let that gas build up pressure. I would not expect to see a tiny 4" barrel benefit AS MUCH as a 5"+ barrel. The MK23 has a great long barrel...so that's good.
Now, the valve is held in the starting position of open by a little spring. If that spring is too soft, the valve will close prematurely (relative to getting the optimal amount of gas down the barrel for max FPS). If that spring is too stiff, the valve will remain open and the mag will simply vent all it's gas, never starting the recoil cycle. So there's a bit of a balance between the spring and the valve. Personally, I want a pistol to function 100% of the time...and if that's on the safe side of squeezing out 1 more FPS that might cause a failure..that's ok. Part of that reliability functioning is making sure I have enough gas to shoot all my shots out of the mag....rapid or slow fire. Rapid fire will cause a great amount of cool down in pretty much any pistol mag...some handle it better than others. Again, the Mk23 has a nice big gas resevior...so that's good.
So...know you know how the valve works...what to do? How do you figure out what's the optimal balance? What part to replace/swap/upgrade?
General rules of thumb when fiddling:
1. Change one thing at a time
2. Chrony to be certain, everything else is just guessing
3. Keep the temp and fill of your mags consistent (usually means topped up and skin temp warm).
4. Disregard the first shot of a fully gassed mag...it'll usually chrony a bit lower than the rest (gas pressure is really high in the mag after a full fill...evens out after there's some expansion room)
Personally, I'd do the following in this order...'cause (IMHO) it's the easiest/best blend of bang for buck without massive headaches:
1. Tightbore and hopup rubber...for mild FPS boost but mainly accuracy (gotta still hit what you're aiming at)
2. Recoil spring...for quick follow up shots and action (it's fun)
3. Nozzle and o-ring/piston seal...for durability and optimal recoil
* maybe a light weight blow back at this point if it's a hicapa
4. HiFlow knocker valve in the mag(s)...6-15fps boost with a change to a part that's pretty much guaranteed not to screw up your pistol. Downside is you need to do each mag
5. Heavier float valve...personally, I have seen some improvement in some guns...nothing in others. I've seen them cause reliability problems in some, and resolve reliability issues in others.
6. For hot summers I'd put in an enhanced main spring....
* if it's really hot, most stock knockers are going to take a pounding with an enhanced main spring...so I'd consider a steel knocker...which will lead to wear on a pot-metal hammer...so maybe a steel hammer...which will lead to wear on a pot-metal sear...so maybe a steel sear too.
So...hopefully you can tell that some "upgrades" are relatively cheap, easy to do, don't really affect the balance of other parts of the system. Some upgrades result in higher FPS....others don't. But to support how you get to the higher FPS...some related parts might need to be upgraded.
The ONLY sure thing is this:
Often, the fastest way to ruin a perfectly good pistol is to upgrade it.
If you really, really understood all of the above...great. If you haven't and want to get go buck wild with your pistol, do what Bloodsport does...tell Illusion what you want to end up with, give him fist fulls of money, wait paitiently and politely and let him do his thing.
Best of luck
Tys