Hi Ian,
I replaced my OEM mid pipe with a straight thru one. The original is 2.25 inch diameter and I replaced with 3 inch. See more here: http://www.350z-uk.com/topic/67814-me-and-my-zed-update-151013-littletweaks-and-a-massive-plane/page__st__120
If you're still retaining the original diameter pipe and only removing the mid silencer, don't expect massive volume increases. The sound will still be restricted by the chambered construction of the back box.
I did get a nice deeper sound with my pipe, but think this had a lot to do with the increased diameter too.
If you've got the opportunity to try it and "suck it n' see", then go for it. I was hoping for a bit more shouty exhaust when I had mine done, but TBH was disappointed. Hence the reason it was shortly replaced with a very shouty K1 and the custom mid pipe is now in the for sale section.