I don't know about the DG pipe, but it looks similar to the FMF with it's disc design. I got my Megamax from Rocky mountain ATV for $170 and this thing really added some power. I'm running 8 discs and a .160 jet.
I don't know about running 12 discs and no jetting change. Add more fuel and you may be very surprised. I think a stock jet is ok for 6 discs, but above that, you may be risking your motor.
Also, air in and exhaust out is the same concept, regardless of the muffler providing it. That's not only why I think you need to look at jetting up, but why I also think the airbox mod does not look out back to see if you have an HPD pipe before deciding to make more power. It makes no difference what brand as long as it's a good system. The power band may differ between brands, but if you are letting more air in, the muffler brand does not matter, the CFM does.
Rick told me that there was hardly a difference at all between the HPD pipe and the FMF megamax. I saved a lot of dough and did not have to cut my machine up.

 

Barelylegal,]

I talked to the guy at the dealership and he said the DG pipe people said no jetting needs to be done unless there is a problem. I guess there is a bunch of Scrambler's, that race with this pipe on, that the dealer is somehow affiliated with. Anyway, he said they have had no problems with the engine and have not ever needed to jet thus far. The pipe is just bolted on and away you go. A lot more power and not too much noise! Another Scrambler (1999) 500 4x4 and I are going riding tommorrow for the first time with the DG pipes installed. I am anxious to see how they run!

 

if you buy a pipe that says it doesnt need a jet change than you wont be flowing anymore air makes sense but if your not flowing anymore air then your not making any more horse power
the hdp aluminum silencer and pipe come with a 165 main thats two sizes up from the stock 155 jet. because the header pipe is huge compared to anything else allows more air to flow through

 

Jetting is $2 and a very simple thing to do. They also said I did not need to jet my FMF, but when I called FMF and Dynojet both, they said yup, sure do. You just can't make power without fuel. You might free up a little torque or something, but you will be pushing it in the A/F ratio area. Anything over 6 discs, you need to jet. Put 14 in a DG and I bet you'll burn the electrode off the plug. On a cold day with a K&N you probably will ruin the motor.