Gains could be put into 3 categories: technical, visual/sound aesthetics and purely psychological.

One should keep in mind that all gains are very much relative and in any event most likely marginal.
You might have people around arguing that the "x-pipe" (that is in V&H language Power Duals) "really wakes up your engine" and "you can really feel the difference" and "now its much more responsive" and so on. As people tend to defend

their recent purchases also placebo-effect kicks in. For these reasons, before taking these kinds of "sales pitches" for granted I would ask for dyno results and/or a test ride.
However, something and in many cases the most important thing to consider is noise as the frontpipe could make a worlds difference. I have a friend who has stock (cat) front pipe and V&H HO mufflers (CVO SG 15) and the sound is "low rumble" and definitely not particularly loud. Based on this I ordered (CVO SG 15) V&H x-pipe (Power Duals) and V&H Monster Rounds and not HO's. What I wanted was "not too loud" and no interference with the stereo on the road. Coming from CVO RK 14 with cat + stock mufflers I was shocked of the difference. To tell the truth, my first impression was: these are too loud. The dealer friend was also a bit surprised. Stock front pipe with cats + HO mufflers is clearly quieter than Power Duals+Monster Rounds combo. After two months I've actually started to like them and I don't think they're too loud.
One other thing to consider, if seeking for a bike with the right combo to listen to: there are also reports that V&H Power Duals + HO and/or Monster Rounds combo sound varies greatly between 103 and 110 engines. Some who are coming from 103 engine and have been happy with the exhaust setup are unhappy when they have exactly the same combo on 110.