Same experience here. I've had rear tires worn down to the casing in the center without noticing any difference in grip. Tread isn't needed on bike tires anyway, slicks work just fine even in the rain. A buddy of mine likes to run his rear tire down to the casing, where threads are visible most of the way around. No point in running them that far though really, you're just risking problems I think.
My favorite tire is also the GP4000 Conti's. They aren't cheap to buy, but on a cost-per-mile basis, they have proven cheap to run. (Usually get at least 4000 miles on the rear tires before they wear down to erase the "indicator dots".) And rarely have to remove a tire early due to cuts or damage; they just hold up.
OEM tires: My new bike this spring came with Mavic Ksyrium Elite wheels, mounted with the matching tires. First ride, I encountered some light rain and the rear tire flatted from a stone-chip puncture. That just doesn't seem to happen with the GP4000s. Also noticed the Mavic tires have a "flat spot" from manufacturing, ie, they aren't perfectly round, probably where the casing overlaps. I like tires (and wheels) which are round and balanced.