Here is what I see in the posted charts of Keefe and Dee's cars.....
 The downpipe is allowing Keefe's car to offer good spool up.
The biggest difference where Dee;s power takes over is the way the boost is being controlled, the boost was still ramping and spiking on Dee's car where his power took over. Notice how Keefe is still making more power where the boost crosses at 4400 RPM.
At 4800 RPM it looks like 17PSI Keefe and 21PSI Dee.
(That 4 PSI is HUGE) The 3 inch Cat back is also helping Dee keep some of the extra power after the spike and the dyno roller has picked up inertia as well. The size turbo they are both running gains a lot with every PSI added in this boost range.
The boost on Keefe's car was being controlled and that is where the power curve will turn. If Keefe had race fuel or water injection and the boost was turned as high as Dee's in that area, his curve would easily keep climbing. This is a chart of a breather car with GT28R turbos on 93 octane (BLUE LINE RUN 19 pump) and race fuel (RED LINE RUN10), the reason for the chart is to show how the power keeps climbing when the boost is allowed to go higher. 
The difference in low end response between Keefe and Dee's car is what we what found with the 3 inch expansion pipes over open downpipes and is why we brought them to market.
The open downpipe never shows a gain in any part of the chart when doing a direct comparison.
When we did testing of coning back 3 inch to 2.5 inch vs. full 3 inch cat back, we only had the stock 2 inch intercooler piping that we were testing with.
The trend with the 2.5" intercooler pipes are being added shows the full 3 inch cat back makes significantly more top end with the larger intercoolers and pipes.
What I have seen as clear patterns in the tuning we have performed, is the turbo can move a lot more air and provide a lot more overall power when you remove resistance in the system, as long as you do not cause turbulence. Most of the system can simply be larger, but the downpipe design is a critical piece.
Any parts in the system that cause the turbo to work harder to move the air, cause less air to move, and result in less power. No where does backpressure pride more power on these turbo set ups.
Any pressure or resistance in the system causes a loss of power.
On a side note, I would highly recommend that anyone tuning these cars add a Knock system to the tools they use. It is amazing how much the knock box picks up that you can not hear with your human ears. Ever since the first time we used ours, I can not believe we ever tuned without it.
In my experience, that spike on Dee's car to 21 PSI is more than likely causing knock if water injection was not used.
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