Overwatch on the Long Rifle (with an Infra, ideally) would lock down lanes much better than Overwatch on the Flamer, which would be better suited to having Infiltrate or Hip Shooting (Though sadly you cant get the latter, since Delaque lost primary shooting).
You may as well just get Photo-Goggles on your Flamer if you're just wanting it for the ability to shoot through smoke, since flamers auto-hit anyway (and to my knowledge you don't reduce the bonus save cover grants to template/blast weapons with an Infra-sight). You'd be saving 15 credits immediately, and have pretty much identical performance.
Some house-rules let you just shoot into smoke with a template weapon anyway, since you don't actually need to target anything specific.
Incidentally: There's no reason you can't overwatch through smoke with an infra-sight or Photo-goggles. Neither require an "aim" action, they're just worded as allowing the fighter to shoot through smoke (With a specific weapon in the Infra-sight's case, and with any equipped weapon in the Photo-goggles' case).