They’d be a lot slower to the point they wouldn’t be able to keep up with, say WS or BH anymore where right now all the classes who specialise in ranged damage + IB have a really tight balance (with flame weapons having a kinda unimportant numbers advantage if the director spawns more/larger hordes).
Yeah I originally used Hunter as I was trying to improve boss damage and blast output but found crits too unreliable than switched to Heat Sink, which was ok but still not reliable enough to make a real difference, then ultimately settled on Thermal Equalizer as it’s reliable, consistent, and you always get the full value. Used properly right now, even with 50% overheat reduction, you’re still venting your Pistols constantly taking a lot of vent damage that you’re offsetting with temp health and NB (you still have to be mindful though). It slows you down too much when hot to not vent (though I see a lot of IBs try to play that way). It takes very cowboy twitchy style shooting and there’s constant dodging, twitching, and positioning involved with both the shot and the blast in horde environments. Factor in that min:max demands a lot of weapon switching + melee on top of the management considerations and it results in a very busy and skill oriented playstyle on an otherwise very slow/bland character. Effort:output, it feels just about perfect to me.
On Sienna, when people talk of never having to vent it’s because they’re using “free vent on ult” instead of 30% Ult reduction. It may be a lil cheesy but, output wise, it’s pretty bad and wasteful. It’s kinda like using a shield, in its own way. Just kinda impotent. With just Heat Sink + maximum crit rate, your output is much better (especially where it counts since you need Head to address a lot of situations when using a Beam Staff) and you do still have to vent a great deal (especially considering her small health pool), so the management aspect is even more intensive than on IB. Can never trust RNG to keep you alive (read:not self-immolated) and Heat Sink ultimately just keeps things interesting by insuring your tempo is exciting and that you can keep up with other classes. Pyro just has a really low skill floor and allows for an above average skill ceiling with a lot of versatility.
If you haven’t, it’s worth trying Sienna out in VT1 to get a feel for what I mean. Even with Haste and HoD, for example, she’s just soooooooo slow compared to everything else in the game and the amount of heat management is a lil obnoxious bordering on unfun. W/o Heat Sink, we’d be worse off than that due to the lack of any other consistent mitigating elements and the class’d be a real slog, imo.