SNSD and DBSK would have been phenomenal, if they'd debut now, they'd have seen explosive growth within the first year.
DBSK's high vocal/dance/performance talents were extraordinary, SNSD's vocal/dance/performance talent would be on a high level too and they would be completely unmatched by any 3rd/4th gen group on the variety level easily winning the hearts and minds of Korean general public.
Big Bang would most likely do very well too, Wonder Girls too.
2NE1, not sure. People have gotten used to girl crush, so they wouldn't stand out for that so much, and I feel K-pop fans would have started to expect different visuals and looks from an YG group now - the bar on visuals has been raised in general since 3rd gen and 4th gen.
But if they had Teddy's songs, they'd do well.
T-ARA, Sistar and KARA, would be slow starters. Only when they'd start getting hit songs, recognition would follow from the general public and K-pop fans. Same for 2PM and Super Junior.
But as soon as they'd start to get recognition and a few hit songs, they'd do well, just like they did in 2nd gen era.