Bushwhacker, I have seen the planetsoarer posting before, it is a Y pipe design, it was guesswork and the results will not be the same for other engines and configurations as total dimensions are important. There is a lot of math involved along with R and D in developing effective systems. Sure, you can get lucky with a "new" design but I would say few improve upon the stock Y pipe on these cars.What about this merge design ......Would it be effective on the 1UZ?
Simple enough to make in a shed.
On my Hilux conversions, I use the stock LS400 exhaust manifolds into 2.5" down pipes (about 12" long) then the rest of the system is all 2" pipe with a 'H' pipe where the 'Y' pipe would have been on the LS400, then a Flowtech Raptor muffler on each end with polished 3" tail pipes. No Cats. Nice sound to the exhaust at idle and light throttle and an 'I take no prisoners' bellow at WOT!
I am unsure of why you use 12" sections of 2.5" pipe off the manifolds?
If it were mine I would make a smooth transition from the manifold outlet to 2.00" tube.
Why an H instead of a Y or X?
BTW, there are multiple types of X and Y pipes and they function differently.