The fact that they are low profile doesn't change the way they are measured which is the same as any other tire.
The penny test is still valid and just like any tire that is legal for sale in the USA these tires are equipped with wear-bars to show when the tire is due for replacement.
As a side note, as long as the tires are reasonably fresh, typically less that 2-3 years old (as the tires age the rubber stiffens reducing traction), dry traction is actually enhanced as the tread gets thinner due to better support of the tread blocks and because the thinner tread generates less performance degrading heat. Wet traction, on the other hand, benefits tremendously from a deep tread that allows for superior water drainage to reduce hydroplaning and improve tread drainage. Recent tests (sorry, I can't remember which car magazine) indicate that wet weather performance declines significantly in the second half of the tread life due to the shallower treads' limited ability to flush the road surface of water.
In your case you'll probably change tires before they wear out because they tend to get very noisy in the second half of the tread life.
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