I'm running a water to air setup that seems to work decently. 2-1/2" is what I'm using and it seems fine to me. The HX35 outlet is ~2-1/2. I think most original (banks, ATS, etc) turbos are ~2" outlet, but I'd still use 2-1/2. Easy to get a adapter silicone coupler.
How did your IC setup 'fail'? Unless you're running something off a lawnmower it should be fine - really not much to go wrong there as long as it physically fits. Harder to optimize, but shouldn't be hard to get something that at least does a bit.
The Hypermax plumbing design puts both pipes to the right hand side of the truck. The first IC I bought, was a same side unit with 2.5" inlet and outlet. The Hypermax pipes are 2.25, so I bought reducers. The physical size of the IC was fine but I guess by design, was very restrictive. I took off down the street and had zero boost, lots of rattle and tons of black smoke.
The second IC I bought was an end to end unit but was probably too small. Keep in mind as I mentioned earlier, the supply and return pipes are on one side of the truck, so I had to use an IC that was short enough, to be able to do a U-turn out the other end and back to my return pipe. The second IC was 25" wide but only 7" high. It worked ok but no change in power and no drop in egt's, just dont think it was large enough.
I need to keep my width in the 25" area but also need to find something quite a bit taller. I have all the parts laying in the garage and whenever I find the correct unit, can have it all put back on in just a matter of minutes.
The Hypermax intake hat was not cheap by any means, nor was the plumbing, so I hate to **** can the project altogether, just need to find something that will work.