Nobody should disregard NP Thundurus-T because its a very real threat now. Yes, the speed drop means it can't sweep through some of the fastest offensive pokemons in the tier (fighting trio, lati twins etc) but it's still an incredible wall breaker. +2 Focus Blast just hits so hard, and kills most of the Pokemon i've seen people use as "checks" to deal with Thundurus (Tyranitar, Ferrothorn, etc). It's not like base 101 is slow by any means either (though you're less likely to sweep offensive teams now).
The best part is that unlike the original Thundurus, Thundurus-T is immune to electric moves, which means that defensive jirachi's that rely on thunders to stop you are fucked and T-wave Blissey / Ferrothorn are limited against Thundurus.
Give it a chance, you won't be disappointed.