Welcome to AA.
Well, I haven't used a pistol as a primary in a game, but Wolf has. It wasn't even a gas, it was a TM MK23 <b>spring</b>. Even then he got a few kills, so, yes, if you are good/experienced enough (with your pistol), then I'd say it'd work as a temporary primary.
Well, my friends and I see a problem with upgrading right off the bat. One, you tend to lose appreciation of stealth, and other skills needed to get in closer for the "stock" kill. Two, you could be putting yourself in a world of hurt because you don't have a back-up, stock weapon. So, if your upgraded gun breaks down, you're stuck. Also, I wouldn't say it's more efficient because:
1) You pay for labor costs when you get a proffesional installation.
2) You don't get experience taking down your gearbox/gun.
3) Because you don't get experience taking it down and repairing/lubing/cleaning it, if it breaks down, you won't know how to fix it, and no one wants to send a gun all the way across the country, or, even to HK, to get it fixed.
In summary, yes, you can use it as a temporary back-up. No, it wouldn't really be any more effecient to get an upgraded gun without doing it yourself, especially if you only have a gas pistol. It wouldn't be fun playing with that as your only gun, especially if you had a choice in the steps leading to it. Sorry if some of this sounds like babble, but I'm pretty tired right now.
--Cyphen