First of all, I'm not a firearms expert, I'm a hobbyist, enthusiast, whatever. Like a lot of you, I've worked on my own guns, but done so with an abundance of caution.
All of this that follows is just my opinion. Take it for what it's worth.
I see many, many posts about using aftermarket mag extensions with the G43. +1, +2 sometimes even +4. I have experience with the G43 as I own one. I also see similar posts about the G42 (I don't own one), and sometimes the G26 (I do own one, but we'll leave it out of the discussion).
I *always* try to dissuade people from using anything other than stock mags in their G43 (or G42). My experience with mag extensions in the G43 was that it runs fine with round nose ammo (and still does) and jammed on the last round with hollow points. People here on GT have isolated and solved this issue (I believe) by modifying the ejector (not the extractor) to stop the ejector from canting the top round to the left. I applaud and appreciate this effort, but still do not recommend using mag extensions.
Often times you'll hear someone say "I've shot 1000s of rounds, no problem". Well I'll betcha a nickel it wasn't 1000s of rounds of self-defense hollow points. A 1000 rounds of round nose does not mean the gun will run with hollow points. I too run mag extensions on my RANGE mags with round nose ammo, and they NEVER jam.
When your G42 and G43 was designed, it was WELL tested by GLOCK with the mags it ships with. No, your individual gun and mag wasn't tested individually, but the design was. I'll also betcha a nickel that GLOCK tested with FAR MORE rounds than any aftermarket mag extension company ever has. Note: I don't *know* how much GLOCK tests or how much <insert aftermarket mag extension co> tested - but I'm guessing that GLOCK has far deeper ammo pockets than whomever made those mag extensions.
So if you're walking around with a +1 or +2 mag extension and it's not well tested, and you need your gun, and it jams, you'll have to clear it. That means... ejecting/clearing the stuck round (-1) and taking the time to do it (what's that worth?). Was that +1 worth it now that it never got used and cost you time?
Finally, the beauty of the G42/G43 (and G26) is the shortness of the grip; the grip is the part of the gun that's hardest to conceal. I see no point making the grip of a G43 as long as a G19.