I've been considering the underneath process for some time, now. In relationship to the changin out the rear two(6 & 8, in my case) on the pass. side. I've herd it wroks well. Just haven't gotten around to it, yet.
I've done it from the top on my now totalled '98 (M6) T/A. But, it was a bear to get to the rear two on the pass. side and required a couple of ext. and universals. Gettin the new plug(s) back in is a breeze. As long as it's threaded properly(w/ some anti-seize on the threads) and NOT cross-threaded going back in. It's just breakin the ol' plug lose that's the killer.