Thing with Powerflow and Longlife is that they operate on a franchise arrangement, so different branches vary wildly. Even if they didn't, it would all basically depend on the fitter who fabricated your exhaust.
I've heard some fairly varied views on Powerflow particularly. I know a guy who had a 16v done, quite likely at the same branch you're considering - he lived in that part of the world - and they made him a system with a mid section that resonated anywhere above 3000rpm (just where you don't want a boomy resonating exhaust in a valver!). He gave them a good few chances to sort it and they just couldn't manage to improve matters any. He didn't stick with it for long, I don't quite remember what happened about it all in the end, but he certainly had a few less than complimentary things to say about them.