Just find what works for you and go with it.
I carry a XDm .45acp 3.8" compact (similar size to a Glock 30) and I use the Pierce pinky extension on my 9rd mag (13rd mag has the grip sleeve). It is just more comfortable for me to have a place for my pinky to go and does not effect concealment at all.
Using your middle finger as the trigger finger is a bad idea, learn to use the weapon as it was designed to be used. I too imagine the slide cutting you with that high of a grip.
Silver... Pierce makes both kinds of mag extensions. The one Hellbent has does not change capacity, but they do make others with +1 or +2 capacity like you have.
Honestly none of the Glocks are good for concealing.
Tens of thousand of Glock owners would disagree.
Whoa. I'll apologize right now, I read your post the first time way too fast obviously. If you're main goal is concealment than I *HIGHLY* suggest you forget the Glock 21, or any full sized handgun for that matter. Remember, if the goal is concealed carry than that is primary: you have to be able to hide it. Period. You cant hide a G21, G17, Sig 226, Beretta 92, 1911, etc. They are too big unless it is the dead of winter and you're wearing a parka. Instead think *small*, think Walther PPK (James Bond's pistol), think Sig 232 (my personal choice), Ruger LC9, or at the most a Glock 19/23 or Smith and Wesson M&P compact. You want something you can *hide*, shoot easy enough so that you'll practice and practice some more (the .45 ACP, and even the .40 SW are heavy hitters and are not kind to light compact guns, fatigue is an issue!). Think 9mm Luger or .380 ACP for caliber. 
I disagree, while I usually carry the XDm compact, I have found that with its thinner frame a full size 1911 conceals just as easily, and I know there are many folks who carry them daily. Also if you carry IWB barrel length doesn't really matter, and with a little cant to the holster a slightly longer grip does not make much difference either. BUT... it all depends on many variable, body shape, how you dress etc.
You are putting everyone into one category here. People are different, they have different body sizes/shapes, dress differently, different strength levels, and different skill levels. Sure for someone with a thin build and scrawny arms a .45 may not work, but I can shoot either my XDm .45 or my 1911 .45 all day with no fatigue, and I can conceal them both easily. As for caliber, smaller DOES NOT = easier to shoot or less recoil, there are to many other variables involved. I much prefer the recoil of my XDm .45 (shooting 230g standard pressure, tried some 185g +P and that's a whole 'nuther story) to that of a Ruger LCP .380 (shooting standard pressure 90g) the little .380 is just too small for me, the recoil is bad (not more powerful per se, just "bad" for me) I can't keep a grip on it between shots, and I can't hit squat with it. I'm NOT ragging on the LCP though, just making my point that there is no one size fits all. I have a friend who carries the LCP and loves it, it just doesn't work for me. Smaller caliber does not = smaller gun either. Think Springfield XDs (in .45acp) VS. Beretta 92f (in 9mm)
There is no
"one size fits all" when it comes to concealed or self-defense handguns. Each person has to reach their own conclusions as to what works best for them, be it revolver, semi-auto, full size, compact, sub-compact, caliber choice, hammer or striker, etc...
