Paul
Mate I have the HKS 2.8 Step 2 plus -5's , Hypertune plenum and have over 600 plus hp.
Running standard Nissan but custom match ported and polished exhaust manifold by HKS , plus Tomei dumps.
Also heavily modified head with 264 cams with all work done by HKS and Naprec .
The car has stacks of response.
They recommended me stay away from HKS & Tomei manifolds as they have issues with cracking. Not all of them but a lot do.
Also told me not to use the HKS dumps but go the Tomei. My engine builder there does not try to sell you company products but what he knows that works and has no problem giving the thumbs up to other brands.
He also is big on running standard manifold but match ported and polished on anything up to 700hp. On big hp engines he makes custom pipes.
Funny here a lot of people in Oz keep going on about the old Tomei dumps due to the fact they are larger diameter but the new type are different design and made of different material.
The new type are designed to flow better and faster than the old larger diameter design.
I still have a secret part to install which will give me another big hp shot. Its olive black, weighs about 9kg, good for 2000hp and vibrates like a vibrator on crack !!