Most Tornadus-T don't run Heat Wave, and a lot of Thundurus-T don't run Nasty Plot. NP Thundy-T has no counters anyway, except maybe Unaware Quagsire. (Or maybe Lightningrod Seaking? lol) (EDIT: Seaking can't take a boosted Focus Blast, but Thundy needs +6 to have a chance to OHKO with HP Ice). And while you're Heat Waving Jirachi, it uses Body Slam/Thunder Wave and paralyzes you, and haxes you to death.
Anyway, I've been running Slowbro+Amoonguss alongside Heatran, and in my opinion it's even better than Slowguss alone. At this point it really needs a Ground immunity and a way to handle Tornadus-T, but it's still solid.