Quote from the guys at Toomey Racing:
The TR6 pipe design, is for the stock engine specifically.
Anytime you modify the engine where you will be altering the exhaust gas temperature, (Exhaust Port change, or compression ratio change) you will be deviating from the TR6 design capability.
The reason it is the best, and works so well on a stock engine, is that it is a VERY specific design meant to maximize the potential of the stock engine, which most people actually have. It is not designed to accommodate any other port design range at all.
Our T5 design DOES have this wider design capability, thus it works well on stock OR modified engines, right up into the higher RPM ranges as well.
But, not AS well on the lower RPM ranges, at which a stock engine/RPM range operates, as the TR6.
So, for low-end and mid range power, in the stock engine/RPM range, the TR6 is your best choice, and for modified engines, or sustained high RPM operation, the T5 is the best choice.