The only reason you would want a dual 2.5" or larger would be if you are using a positive displacement supercharger like a whipple, sprintex or eaton. Otherwise 2.0" or 2.25" is as large as you need and want.
There are some Japanese and Chinese companies that build/built Lexus systems but most are just mid pipe back. The SC400 is an older model and I cannot say if any are available.
