the retract body goes into the duct just a tiny bit, protruding into the duct by about 3-4 mm. The degradation in performance is completely negligible, as you can barely see it when you look into the intake.
It took me about ten evening to actually do the whole retrofit. But I had been thinking about it for several months, just thinking about placement and proper retract mount anchoring etc.
Twin or single is irrelevant, basic physics tells you watts is the only thing that counts when you have good ducting
. The single has an edge in ease of retract installation. I personally prefer the sound and performance of the twin.