[TurboKraft]

Tippy: you will maybe hear some of those 993SS cams' lope with a good exhaust, but it will mostly be minimized.

Part of the beauty of ITBs is they make running really big NA-style cams possible on a turbo engine without having the reversion on one cylinder mess up another cylinder (as happens on a single throttle manifold/plenum).

GT2 cams are suddenly small when running ITBs, as are SS cams (basically mechanical version of 993RS hydraulic cams).

3.5mm overlap? Awww, that's so cute... Kidding aside, you can get away with larger than that even and still have a civilized idle at or below 1,000rpm. Our 4.0L Twin Turbo has some big bad Edelweiß cams and she starts and idles with no drama like a plain 964/993 3.6L.

Back to your question jsveb: you'll probably be more limited by limited real estate beneath the intercooler, like Ken indicated. I'd almost start from the top and work back down -- space below the IC for volume for plenums and their flow & plumbing, less the minimum space for base adapters and the t-bodies themselves... then inlets to t-bodies... you run out of space real fast.

Also, at some point you are splitting hairs on a street car. General rule of thumb on 911s since the 1970s was high mounted throttles & injectors for maximum mixture and velocity and peak power (picture high butterfly RSRs) vs. low mounted (993 3.8RSR). Even if you built two systems and tested them back-to-back, I doubt you'd see any significant difference.