The stock castings are not very well designed IMO. I know (I'm pretty sure) these companies spend tons for R&D on design for these things but it seems so simple to provide a nice gentle transition from opening to throttle bore. The OEM (all I've seen) have an abrupt step when the straight taper (cone) reaches the bore size. I was pleasantly surprised when my 57mm TB outflowed an OEM 60mm TB.