If Infiltrator / TC is too easy mode for the players who know how to play them, maybe they need to make Gold harder.

I imagined what multiplayer would be like if the enemies could run and gun as fast and as well as we could. Or if they hid behind barriers and instakilled us.
As it stands, it's hard to make computer-controlled enemies smarter because AIs just aren't at the same level of intelligence as the very best of human players. Instead, npc enemies end up getting significant (and some might argue 'unfair') boosts to their health, shields, etc., or increased speed, infinite ammo, etc. in a bid to make things challenging for players. A pity AIs can't be auto-adjusted on the fly to compensate for players who are clearly too good at playing certain classes or simply too good in general.
Probably my favourite AIs were for Unreal Tournament, where at their highest setting were relatively challenging (although still far from impossible to defeat).
As for balance issues with TC or Infiltrators, while it's not entirely immaterial (it's not really fun for non-infiltrators if Infiltrators can go around soloing the map at the expense of other people's fun - I'm not saying that happens, but I'm saying it would not be fun if it did), I'd say it's far less of a concern in a co-op-only multiplayer game than it would be on a clearly adversarial PvP system where it really would be unfair.
I don't know that it's a good idea to balance a game based on the top 1% of players and what they do, because those people are relatively rare: it's one thing if any Joe Blow could make an infiltrator and using the worst sniper weapon in the game, still solo Gold by using TC. But I don't believe that's the case here, and if it takes a combination of experience, skills, race and class combination, specs, rare weapons maxed out, and a level 20 character to solo Gold, then I don't think it's terribly unbalanced. If there are some players who are a little too good to be using an infiltrator, they can always try using a different class, or try playing on a higher difficulty with a lower level character, or even just try using a crappy weapon for a change just to spice things up. Balancing the game based on the best players is going to make the game significantly harder for the average, casual player, especially the ones who are used to being able to freeze the action and to deliberate on what skills, powers, etc. to use in a given situation, and aren't really ready for the FPS-style pace of multiplayer. I know I found even bronze challenging when I first started, and even now I wouldn't consider trying to solo Gold.