The HKS for R33 is 95mm, but it tapers.
It starts at 3inch from the cat, hits the little hotdog muffler and goes up to 3.5 inch, then after the flange for the cannon it steps up again to 3.75inch.
Its the second exhaust i have owned thats done this. My old Mugen Twinloop exhaust on my ITR did the same thing.
I have the HKS on mine still with a GT30. For the street is a little loud, so have built a replacement middle section with another muffler for the road.
Heres the HKS Vs Apexi. Sorry the pic is a bit small, but you can kinda see the tapers, and that it is substantially larger than the apexi.