Sirius + foregrip?

I have the Sirius but mostly hip fire. I've used ADS in a pinch and it works but isn't extremely effective. Even the Cyclone with grip and comp isn't that great at range. The damage drop off with all SMGs outside of the PDW just handicaps you and you may as well be using the PDW or a carbine if not playing Infiltrator.

That said, it can be done if you're a good shot, burst fire and land some head shots.

Sure, that is a benefit. But having improved accuracy and mobility with ALS is a bigger benefit. A good player can choose not to engage too many targets at the same time and knows how to pace reloads to string together kills. If you're playing an SMG LA or SMG infil correctly, the extra damage per mag is almost useless compared to the extra 1v1 efficiency of having ALS.

The only reason you'd want to use PDW to begin with is to be more competitive outside of typical SMG range, where ext mags only make sense if your aim isn't good enough to kill one opponent with 30 rounds. Because killing one opponent per reload is all you need outside of CQC. If you want to be able to engage multiple enemies at closer range, 2nd gen SMG's are preferable over PDW because you get better DPS, 50 rounds and ALS all at the same time.

I always use extended mags on the SMGs that don't have it "built-in".

Actually, I'm not sure there's an extended mag option anywhere in the game that I don't take. With LMGs that have the option it's literally the #1 selling point for those LMGs for me. For vehicles, typically both the Mag Size and Reload Speed certs result in the same sustained DPS, but the Mag option provides better DPS in the short term.

It's never about needing more rounds to kill a single opponent. It's about having more control over when you want to reload.