The RT Highflows need to be welded, John B at parts rack can hook you up with all the right parts to connect your flex pipes to the Borla.
I wound up buying the Roe High Flows for my GTS since they matched up size wise to the stock exhaust or any other cat back system designed to connect to the stock cats.
They came with clamps, but leaked exhaust pretty bad so they had to be welded.
I guess what I am saying is you can go with high flows, but need someone to weld them in for the job to be done right.