Knowing that DC is made by AEM, Id say the fitment will be fine. Usually when you pay for a high priced item (usually), part of that price is due to the fact that someone actually built it specificaly for that car AND made sure it fit before mass production. Of courselots of it goes to marketing too. My GReddy fits perfect; no clamp/weld on muffler look with the tip too far out hittin your leg on it when you walk by, or too far in melting the bumper and filling the trunk with exhaust.
I have seen some white/light coloredcars with nice looking exhaust that had stains on the bumper above the tip. I would assume these are from the car sitting idle, stoplights, etc. This may be what you refer to as burn marks. I can say that my cardoesnot have these marks after 2 years.
Not too difficult to install if you have some mechanical ability and are able to deal with a few rusted solid bolts at the cat(Im in NH). Occasionaly you may need some elbow grease to get the rubber hangers off(maybe some lube too).

 

Did you mean K&N? Or is that a new imitation k&n? lol

 

I'm gonna install a DC catback tomorrow. Fit and function shouldn't be an issue. I hear they are pretty good.