Just to add a data point (and sorry, I can't find where someone was talking about numbers in the thread)...
For a while earlier this year we were running a 3.x kernel and saw a very modest (1-2%) improvement in overall performance. This would be on a server with 512G RAM running ext4.
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Jim C. Nasby, Data Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net