I never really had a Genesect set up a sweep on me and scarf was easy to deal with. I also barely used Genesect, so I am completely oblivious to its effect on the metagame other than the fact that it was used on like 30% of teams. I also never even changed my teams to accomodate for Genesect.. so yea.I feel like Genesect was at least more metagame defining than Tornadus-T. Yeah, I mean, Tornadus-T was certainly a top threat, but not as big as Genesect. First of all, apart from very few exceptions, Tornadus-T was only used on rain teams, whereas Genesect was used on just about any good team at the time. Tornadus-T gave Jirachi some usage, yes. But Genesect gave some Pokemon completely new sets (or rather, ones we never ever see became viable) like Scarf Ninetales and Shed Shell Heatran (to beat GeneDug). Either way, if Tornadus-T makes it on the list but Genesect doesn't, I would be shocked.
Tornadus-T on the other hand forced everyone to carry a way to deal with it even if it wasn't as popular. Jirachi and Rotom-W might have already fit well on most teams, but being forced to use them AND keep them healthy all the way until late-game thanks to Regenerator is a little bit of a hassle.
However, like I said... I could just be oblivious to Genesect's effect as I never saw it as that threatening.