If the engine lugs in MDS mode it does get quite loud, not a bad loud, just a very 'boomy' exhaust note. This used to happen a lot when I only had ~1000 miles on the car, I'm just over 3000 miles now and I guess the system has learned not to engage the MDS if it's going to lug the engine and just have to kick all 8 in again, it does still happen though.
Cruising around 45-50 in 7th gear and MDS on there is a slight difference in sound, but not intrusive at all. At the same time I usually get the same gas mileage (according to the computer) at 45-50 in 8th gear with MDS off.
In other words, go for it!
And I only paid $834 on amazon with free shipping, it is up to $1340 now. Just keep checking.