Nope not seen it. I did compare some DM Xtreme in Halfrauds with my SS and there wasnt much difference to what I could see and feel. But I guess those in the know know better than me Still cost me less than half what DM would have cost and it did the job, and I still run the stock BOSE speakers so you can tell I'm not into my car audio that much One thing that made a big difference to mine was adding closed cell foam (aka camp mat) all over the SS. Helps stop that nasty airbourne higher frequency noise that the SS cant stop.
Whichever you chose, remember to get a roller and a good knife. Having a good bond between the material and the car is key
At the end of the day, anything is an improvement and SS and DM are good products. With DM Xtreme IMO you get the best but you are into the law of diminishing returns for the price. Is it twice as good? No. But if you've got £3k of kit in the car, it's worth the extra to get the most out of them.
And roller and knife are essential as Chris says.
In answer to the question, the ideal is to 'seal' the door skin, and sound deaden it. With DM Xtreme, you can cut it to shape pretty easily and with a roller get it into all sorts of places and contours. The door panel should be tight when you put it back on to get the best chamber. The advantage with DM Xtreme is how thin it is, so there is plenty of room to get it in the doors. If you're not going mad, I would suggest 4 or 5 decent strips of it on the outer skin of the car and around the mounting area of the actual speaker.