Thanks to all the contributors of this thread. All have their own experience/knowledge and it's an informative read.
I recently organised a GB on the NISMO plenum, 4 in SAU have taken it up, and I expect them to be received soon.
The claim is minimal by NISMO (average 1.8%). As pointed out in another thread there is other cheaper ways to reduce your track times/make power gains.
Dropping $1800 for small gain is for those who choose to afford it and hopefully sorted everyting else out, and some perhaps for bling.
I'm encouraging tuners and drivers who install these NISMO plenums report back. On real on road experience & dyno data, not claims or hearsay.
As an aside I'm interested in those new after market Midori? Gsensor too. It'd be nice to get the signals through alot faster.