Currently netperf is being compiled without demo mode support which is required for interim result reporting (-D). The ebuild should either always enable the demo mode support (--enable-demo) or let user enable it via USE.
+ 27 Sep 2012; Jeroen Roovers <jer@gentoo.org> netperf-2.6.0-r1.ebuild, + metadata.xml: + Add USE=demo to enable reporting interim results by Michał Górny (bug + #304651).