The tighter tolerance claim is a word-of-mouth claim. If the L86 and L83 are built to a tighter tolerance, it would be difficult to explain why GM specifically recommended 15W50 in an L86 during track use.
http://www.stingrayforums.com/forum/stingray-corvette-performance/16925-tadge-s-answer-why-15w50-oil-track-use.html#/topics/16925?page=1&_k=dsq5l5
The reality is that GM balanced engine protection vs meeting EPA fuel economy goals when selecting 0w20 for the pickups vs 5w30 for the Corevette/Camaro SS. 0w20 is nice in the winter, but if you drive your truck aggressively in the summer, your engine would be much better off running a 5w30 oil like the C7 Corvettes.