I'd recommend at least 12k before hitting 400nl, but then again I'm a BR nit. If you're comfortable dropping 20% of your BR taking a shot, then go for it.

I thought the transition from 200 -> 400 was one of the tougher ones, but this was on stars. There are still a ton of fish at 400, but the regs are much better and there is a definite step up in aggression.