It can be show that a better solution exists (not necessarily best):

438 : [ eyesore sleekest steepen spindles baseless rockiest refiners sheered tethers exhausts ]

Yep! That's the problem with heuristic solutions. Unless you also run a full brute force on the same data, you can never be sure that you haven't found some local maxima.

On the dataset above, your 438 is better that my 434, but neither are as good as:

441 : [ rendered spindles squires tethers rockiest 
        eyesore sheered sleekest steepen refiners ]
[download]

But even using all 4 cpus flat out, my brute force solution can just about manage 1/2 a million combinations/minute. Given there are 7.75e9 combinations to try, I'm projecting over 10 days to verify the heuristic for that one dataset. That's too much for that sake of an intellectual challenge with no useful purpose.