I have both versions of the Wipperman 10 speed connectors (campy and shimano) and the shimano one does fit a shimano chain tighter than you would expect. When I looked into this problem, I found this information from C-40, who is a very knowledgable resource on rbr but I'm not sure that he ever posts over here. Anyway, this is his information and not mine.
Originally Posted by C-40
First, there are two 10 speed versions of this link. One is designed to work with the Campy 10 version of the wipperman chain (marked 10 speed) and the other, a narrower model (marked 10S1) made to work with the shimano compatible version of the wipperman chain. Neither is really intended to function with chains other than wipperman.
Now to the fine details. Both the original Campy 10 chains and the Shimano 10 chain have the same width across the inner plates (.151-152 inch). The Campy 10 version of the 10 speed link is built to fit a wipperman 10 chain, but it also has the same width, so the fit is perfect with any of these three brands. There is NOT more side clearance between the connex link and any other link in the chain. There is about a .005 inch clearance between the outer and inner plates. This is easily verified with feeler gages.
The shimano 10 version of the connex link is about .005 inch narrower than the campy version and will only fit the shimano 10 version of the wipperman chain properly. It is too tight a fit for a Shimano or Campy 10 chain.
Campy has replaced their original 10 speed chain with new ultra narrow (UN) models. The width of this chain, across the inner plates is only .145 inch. The shimano 10 version of the connex link can be used to join this chain, but it will have .002-.003 inch more clearance than it should.
I've heard that the link is tight but for some people, it doesn't seem to be an issue.
http://forums.roadbikereview.com/showthread.php?t=72284