My first Walther was a Navy PPQ M2, and since I was/am too broke to buy a suppressor, I decided to see how a brake might work on a pistol. Have lots of experiences with rifle brakes and they work well to reduce rise and recoil if designed right. Some say the 9mm doesnt have enough recoil to start with to make any difference, but I certainly found a noticeable difference. My first brake (pic #1 - before mod, pic #2 - after mod)) was a Custom Muzzle Brakes Tri-Delta. Worked well, but I thought it could use a little more venting on top so I modified it by drilling an extra hole in the top, angled slightly backwards. Even better now. Everyone who shoots it, side by side with my Q5, immediately notices the difference and prefers it to all my other non-compensated pistols. Many competition brakes I've seen have far more top vents than side ones to mitigate muzzle rise more, as opposed to straight (backwards) recoil reduction that side vents provide. I think they are on to something...
My second brake I tried was the Lone Wolf large compensator (pic#3). I also enlarged the top vents as far as I could. It's multiple side vents are angled backwards about 45 degrees, which I thought might reduce recoil more than the Tri-delta - that's a proven design that works amazingly well for rifle brakes. However, I found after firing it, that the recoil and muzzle rise reduction was not noticeably different than the Tri-delta, but still very effective. The problem was the far more noticeable gas blowback into my face, that the Tri-delta did not have. The rearward-angled vents definitely work well for recoil, but my face is just too close to the gas path using it on a pistol. I think it would be a good brake for a 9mm carbine build where that won't be as much as an issue (unless it's a fairly short barrel).
I like the looks of both of them on my Navy, but not everyone will... but there are other options. I've since seen others (Trbci) made specifically for the Walther PPQ's. Since the 9mm is so forgiving, I suspect these designs will probably work well too, regardless of differences in porting geometry. Probably going to feel similar to other brakes - definitely easier, softer to shoot, but may be hard to differentiate differences between them.
My long term plan is to get a suppressor, but for now these brakes are a nice addition, and a real pleasure to shoot - easy to see why competitive shooters use them... Just remember that your gun may not cycle properly with one attached - between the extra weight on the end of the barrel, and the back pressure reduction, you wil probably find a reduced power recoil spring will be needed. For my Navy, I ended up using the "orange" recoil spring from the Q5, to replace the stronger "grey" stock Navy spring, and that seems to work well for me. Got a replacement spring assembly from BT Guide rods with the elongated metal rod for the Q5, and also got their standard length orange spring/metal rod for the Navy once I confirmed function with the original stock Q5 orange spring assembly.
But many variables exist that can disrupt proper cycling - having a worn-in, cleaned, and lubed action is very important to make cycling a little easier. Any excessive friction in the slide/action will effect cycling negatively with brakes, as will weak ammo. Some ammo just won't work well no matter what else you do. You just have to experiment, shoot a lot, experiment some more, repeat... thats what I had to do, but now everything works great and reliably with the ammo I like to shoot, train, and carry with. That is 147gr Speer Lawman, and Federal 147gr HST, reg or + P. Some of the lighter weight bullets (115 and 124gr) didn't work as well at first, but work better now that everything is more worn-in.
Your results WILL vary...
I use Lone Wolf compensators, and like em'. They DO work, and I've not experienced any of the blowback mentioned above. It's been my experience that these things, do what they're advertised to do. The recoil spring DOES need to be matched to the ammo you're using.....weaker ammo = weaker recoil spring.
As for opening up the holes on top. Probably a bad idea. Try this experiment. Put your lips around a 12" long 3" dia. piece of PVC pipe....hold your hand about an inch from the other end of the pipe. Now BLOW. Did you feel anything? Probably not. Try the same trick using a McDonalds straw (before they outlaw em'). Put the straw in your mouth, pucker up and BLOW.....you can probably blow the toupee off the dude in the booth sitting next to you.
Bigger is not always better. And what we're needing, as it relates to the compensator is velocity....not volume. Opening up the holes will decrease the velocity.....THAT's not what you want.
This velocity vs volume thing is also extremely important when designing the intake and exhaust ports/system on an engine. Around town, your priority is on the speed of the air flowing thru the intake ports....this gives you a very responsive engine.....better power and economy. And to get this, the ports are sized a little small. However, if its ultimate power you want and don't care about a sluggish engine at lower rpm's then you want to open up those ports....you want Volume. Combine volume with some additional rpm and now we're going to the dragstrip. The bigger ports are going to allow the engine to breath (take in more air....easier/faster).
Try the same experiment outlined previously, but this time, Suck, instead of Blow. Your cheeks start to hurt while trying to suck air thru that straw.
Bottom line, a choice has to be made...do you want/need velocity or volume.....
I'm figuring the holes in the compensator have been designed/sized to provide velocity. Opening them up will decrease the velocity.....which will reduce the effectiveness.