Some years ago, back .. early 80's maybe, Gibson themselves did a definitive test. Someplace... its online. Used to be on the Gibson site, but that site has been rearranged so much... its hard to find anything anymore there.
Their conclusion was nickel silver covers will cut the high end, just a little. Same for regular nickel plated brass, almost no difference. They did say, they weren't sure it could be heard very well, depended on the original brightness of the pickup, the amp settings (which are generally very scooped or low in the mids), the strings, and speaker choice (which generally only go up to about 6k or perhaps just a tad more). They said, it was a very subtle thing, and the conclusion was, if you think it helps, and you can hear it with your pickup, strings and rig, and you like the looks, by all means use uncovered pickups.