Whilst technically a "Flat 6", the 2012 Porsche 911 GT3 RS 4.0 had 500hp and 339tq from a naturally aspirated 4.0l engine. Pretty impressive.
Does that count???
2012 Porsche 911 GT3 RS 4.0 First Drive – Review – Car and Driver
There are a whole slew of other NA 911s that also have/had more than 350hp.
The 2004 BMW M3 CSL had a NA 3.2l inline 6 rated at 355hp.
http://en.m.wikipedia.org/wiki/BMW_M3