They worked great. Might get to higher rpm with bigger turbos, but to about 5500 they make well over 20 psi if needed. The headers are made out of 1 5/8 water pipe (.120" wall), so it's most likely not an ideal configuration. The engine is in a 2017 Charger and has run 5.6 at 130+ mph in the 1/8. It's lighter than stock though.
One single HX40 on my 2000 Ram was good for 40 psi , I could get 44psi out of an HX35 but it was outside it's efficiency map. The larger cross section of the exhaust housing did make the HX40 a little laggy compared to the HX35 but the EGT was lower and there was less black smoke.
