I seem to have problems doing these in groups. The spawns tend to take much longer to work through, so the 30sec bonus that you get for tearing up the PPD spawns doesn't cover the time you spent killing them. Also, the mayhem bonuses for destroying cars, objects, etc seems to take more to actually effect the timer. The net effect is that Mayhem missions in a group are harder because you wind up having less time from having to kill so many PPD and so much extra stuff to get that time. My elec/elec brute finished with 10 minutes to spare solo at level 6. My thug/dark MM with a group of 7(!) failed, because the hordes of PPD and items we destroyed did nothing to boost the clock at all. We had one meter bonus and 6-ish evaded arrest bonus - no cars, no hydrants, no mailboxes, barrels, anything. Combine that with 2-3min loadin time (for some people), large PPD spawns that took a couple min to fully kill, and a few deaths, it was nigh-impossible.
Also, for some reason after we killed the end mob spawn, we didn't finish the mission; I'm not sure why. Some of our members were in jail, so I'm thinking part of the hero spawn showed up to attack them...but if so, its *highly* annoying.
Oh, and I dunno if they are, but exploding items killing other items should definitely count toward your "timer increase" award (Hopefully, they do).