It ain't nothing to "suspect" - they will cost a lot of AP and ammo, it's clearly visible in the trailer. Besides, how couldn't they, it'd be way too OP.
Expertise, i recently made a character with, 5mm SMG user - all the proper feats, and lemme tell ya, on Normal it'd be just fine, but on Dominating, it's not too fancy - lots of bullets each dealing couple dozens extra damage looks powerful on paper, but when you meet enemies with high mechanical resist or extra high treshold, it ain't. This won't change for miniguns any much, even with 12.7 caliber; and as usual, spraying W2C ammo on every corner always ends up way too expensive.
Why grenade launchers "heavy" guns though? Depending on model, they can be both heavy but also very lighweight, in real life. Mk47 Striker, full-auto, weighs ~18kg - but there's also the famous M79 (single-fire, same 40mm grenade) which was used in Terminator 2, which weighs ~2.7 kg empty. Ain't "heavy" by no means - some SMGs and most assault rifles are heavier that that.
P.S. And about enjoying playing a Sniper build - man, you and me both! I played quite a few variations, and i think i ended up figuring no less than the most powerful build the game can possibly have. I mean, more powerful than all the Psi, melee, AR, energy pistol and all kinds of hybrids i am aware of. I was thinking to post it - my sniper build - in the "builds" subforum, but so far i saw little welcome for longer posts i do, so now i'm in doubt if i should. What you think? Should i?