I would be curious to know what turbo you were running during this swap? Also, what MPH gain that came along with the lower ET?
You seem to be gleaning the Right info from this thread, as I would tend to agree with what your concluding. Although you Could stretch the 1 piece a little further than 250HP
Something to keep in mind; Most are going to go off of their Own experience in these matters and many have different experiences, specially when it comes to boosted applications. (boost can mask many things)
Back in the day we would swap 2 piece intakes on to 20psi (stock TII Garrett turbo) 2.2 applications and gain 2-300rpm peak power, which I thought was a significant return for a simple change in stock intakes. So the 2 piece became part of the recipe for us very Early along the build trail. Being RPM conscious, we mostly stuck with the 2.2 as a build platform as well.
When the Charger was running the 57 trim stage 3 turbo at 33lbs boost with fully ported 2 piece and 52mm TB, decent ported +1mm G-head, 2 stock I/C's y-piped with 2 1/2" hardpipes and stock 88 roller turbo cam. I built a custom 1 piece large plenum short runner manifold with 58mm TB and swapped it on with no other change.
Now, I had Just solved my spark blow-out problem with Aurora ignition and .028 gap, everything was working tickity Boo. I still remember how tinny that manifold sounded, like the valves were opening and closing right in front of me. First time I put it down it was lean, so I started adding fuel. (just rudimentary at the time with AFPR) Ended up adding 5-6psi fuel pressure to get the AFR's back in line @ 33lbs boost and was instantly blowing out the spark again, so I started closing the gap on the plugs. I believe I closed the gap to .018 - .020" to stop the spark blow-out and the next trip to the track the Charger picked up 3-4mph.
That was a Very Significant change IMM and I took it as a 30-40WHP gain with no other change. (also ended up going with a Stronger aurora ignition after that lol)