I have been offered by SEA-LECT Design the opportunity to test both and have been doing so for a while.
In my paddling environment (warm waters, think Florida) the waterproofness is equal in both Performance and Classic.
The difference between the two is installation pressure needed to pop them on: less on the Performance ones.
I don't have a preference for either in the big oval size (that fits perfectly on Valley rims) but I do like the Classic better for the day hatch as it pops on with a more decisive spring action and does not need to be checked if popped on correctly.
For a full report on SEA-LECT Design Classic lids check: http://gnarlydognews.blogspot.com.au/2012/06/gear-sea-lect-designs-hatch-covers.html
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