Getting power w/ your left hand/trad grip is a matter of practice - as everything. When I started drumming some 1.5 years ago I discovered drummers like Dave Weckl (I knew him before but wasn't really interested in drumming at that time) and Todd Sucherman - now they play with more than enough power to convince me that trad grip doesn't lack in power. So I had no "and then I saw..." scenario. But I've been practicing both grips and like both (trad grip makes about 70% of my practice). To me trad feels better (more ergonomical and comfortable) for busy playing (e.g. w/ mini Moeller and/or accenting), ghost notes and embellishments. Trad takes some time to get into though, but then it pays off.
Seems to most drummers it's a feel thing, the power with both grips is about the same. Personally I like to have both playing feels at my disposal so I'm practicing both.