I was forced to enlarge my exhaust from the standard cast iron to a 8" dia. opening when I repowered to 3126 cats. My experiance so far has been very positive. They are very quiet and were fairly easy to install.
Bought them through Greg Gajak who might lurk on the Roamer site but is a regular contributor to the Commander site.
Biggest issue is that the f-glass needs to be a safe distance from the mixing valve to keep heat below a certain temp. In my case I needed to keep the fiberglass seperated from the mixing valve by 12" of wet exhaust hose. The pipe and muffler is normally barely warm to the touch when underway. The folks at Centek were quite knowledgable and helpful.