couple of thoughts here:
1. yea, that little hy-9 is pretty maxed out at stock power levels. With the turbo already near the edge of its efficiency map, additional boost is likely to have a diminishing returns thing happening. With that really small e housing, 40 lbs of boost just doesn't give the airflow that it would with a larger turbo at the same boost level. all you're doing is imposing higher working pressures on the system with only small increases in actual air flow out the tailpipe. Additionally, higher boost pressures outside the efficiency map of the turbo tend to generate very high compressor temperatures as a consequence -- so from my reading you're not likely to have an "wo baby" experience from the elbow and higher boost pressures with the stock turbo.
2. One of the ways they produce that textbook looking flat torque curve is via the wastegate, which delivers a constant air pressure to the intake system over the rpm range at WOT. With the 305 HP engine and that little bitty hy-9, my guess is that they've eaked out just enough boost (wastegate setting) to make the numbers, and yet they've set the overboost condition to be only slightly higher than the wastegate setting. to protect from things blowing apart. hence the overboost code with the elbow even with stock fueling. On the 2nd gen hx turbo (6-speed) there wasn't enough fueling to generate higher boost (there was more margin in the design) so the elbow was less likely to set a code. With the 3rd gens it is more likely that there is enough fueling to generate higher-than-stock boost pressures, especially with that tiny little 9 cm housing. think of blowing through a straw versus a garden hose. pretty easy to blow hard enough into the straw to generate high air pressures, but it takes a lot more air to do the same in the (larger cross section) hose.
3. I expect there is some truck-to-truck variability in the actual wastegate operating pressure and behavior, just due to mfg tolerances and process variabilities. THus, I'd expect some trucks to set a code with the elbow and stock fueling, and others maybe not. I get the sense that they've tuned this up so carefully that the system is near the hairy edge of operating efficiency. The e housing is so small and the ECM has enough fueling available to trip the overboost code without a properly operating wastegate in there.
4. There is bound to be truck-to-truck variability in the turbo pinwheel balance. spec is 3% (piers adjust for zero percent), which means that the pinwheels are (or could be) out of balance and will tend to fly apart at high rpms generated by higher boost pressures allowed by the after market wastegate setting. I tell ya, you're toast if your warranty coverage gets any attention.
I'm really surprised that they haven't denied warranty coverage. They still may. all it takes is someone suspicous or passionate about sticking it to you, and they'll have some forensic failure analysis guy discover that you had an after market device in there that artificially allows the turbo to operate at higher boost pressures than they support under warranty. production holests are just not made to spin very fast. bbbbbbbbbbbbbb
Attention: TDR Forum Junkies 
