I actually feel like the infantry are very well balanced against vehicles. Especially the anti-armor infantry.
Gunner is especially useful against vehicles. Much more than he is in the original Renegade, but not over powered enough to make it unbalanced. It fits right into the game and makes him more useful.
Grenadiers are 'explosive' weapons, yes, but they cost 0 credits. You really shouldn't expect them to do a substantial amount of damage to an enemy tank. However, they are pretty effective against other infantry.
You also mentioned something about the early game pace. I actually like the speed. It's not really supposed to go straight into action. It's like the calm before the storm. If you think about it that way, it's actually a bit too fast. The beginning game is the time were you quickly discuss tactics with your team and try to organize a force. I think speeding the game up during that time will do more harm than it helps.
Also, there is one thing I want to add onto what you were saying. The shotgun is hard to read. However, despite that fact, it doesn't have enough power. In Renegade, if you got up to point blank range of an enemy (his back is turned or he is afk or something) and shoot him with a shotgun, it should kill him in one to two shots depending on the character. In Renegade X, t takes like 4 at point blank range to kill a free character and it doesn't feel like all the shots are hitting. It doesn't necessarily need visual feedback, more like an audio feedback.