Most people at 2+2 think that 2-3 bb/100 is the most you can win at limit holdem. But there are a small number of players doing better than that, in the 4-6/100 range. And that's per table, not multi-tabling.
So, from a bankroll perspective, if you can win 4 bb/100 at limit, there's basically no reason to ever play NL, because you would have to win 15-20 bb/100 with the same bankroll. There may be a few people that can swing 15 bb/100 at a low NL game, but it's damn near impossible.