I did this for the first time in 2013. My employer's plan has a fixed $20k limit on after-tax contributions, rather than a %-age cap as yours does. Everything has gone smoothly so far ... though I haven't received my tax-statements yet, so we'll see.
I'm not an authority, but one thing you said wasn't familiar to me. My plan also supports "in service" rollovers, but they need to remain within the 401k plan as long as I'm employed. So in my case it didn't involve an IRA at all. The money is still in my employers overall 401k plan, only now it is being treated as "roth" (rollover) contributions and is growing tax-free.