I actually don't care if employees share salary information - those that want to share are, as far as I'm concerned, free to do so. Generally, I think the best compensated employees will be the least likely to share because they realize less productive employees may be jealous of them. Generally the least skilled workers seem to think they are much better than they are.
I certainly would have no interest in participating in "collective bargaining" for my job. Nor do I think hardly any of my coworkers that I've respected would be interested in doing so. However, I have no problem with others participating in collective bargaining. If a group of employees got together and wanted to bargain collectively and I made policy, I would urge them to try to unionize. But if they didn't want to, or couldn't get enough of their coworkers to vote to unionize, I think I would generally be happy to evaluate each employee in the group, determine what raise I thought each deserved, then distribute that raise pool equally (as a percentage) among the group. I would share with any employee that requested what their contribution to the common raise pool was (obviously I could not give that information to other employees though). Of course, no employees have ever requested collective bargaining in my organizations or in any around me so I've never had to actually face this question.
If you conducted a anonymous survey among all the employees in your statistically significantly sized group (perhaps Department, perhaps below) and asked "Do you believe, among employees in this group who perform similar jobs, that you perform ABOVE or BELOW or RIGHT AT the median?", do you think the number of people who respond "BELOW" would be statistically the same as those who respond "ABOVE"? I strongly suspect that more will select ABOVE than BELOW.
Generally in the US work environment, I've found that low performers are much more likely to overestimate their skills and contributions while high performers are more likely to underestimate or accurately estimate theirs.
Interestingly, some studies have suggested that
the least competent performers inflate their abilities the most; that the reason for the overinflation seems to be ignorance, not arrogance; and that chronic self-beliefs, however inaccurate, underlie both people's over and underestimations of how well they're doing.
An example of this I recall outside my direct field was an acquaintance who was a lab tech who seriously believed that she did the same work as the engineers but wasn't paid as much. This was completely ignorance on her part -- she had no idea what the engineers did outside the lab because she didn't delve into what engineers were doing the other 95% of the time when they weren't in the lab.
As far as the risk I will never get a quality employee who feels like they have been pushing themselves too hard etc... I don't see why requesting salary information would affect that significantly. I would want an explanation of why they were interested in taking a pay cut to work in my organization. Reasons like "more interesting work", or "bored with doing the same old work" might be very legitimate if they were clearly overpaid in their past job (which sometimes happens because someone has become the critical expert or has a critical skill that the employer is so terrified of losing that they effectively "bribe" the person to stay but the person isn't happy and, likely, isn't even doing their best work due to poor morale).
However, in most groups I've managed I probably would not accept the "pushing myself too hard" unless the person was also dropping to a job with less critical responsibility. Outside of a couple employees I've had that were techs rather than engineers, everyone in my group has been on salary so working "less for less salary" is too difficult to manage. As well, most organizations I've been in/managed had ultimate "buck stops here" responsibility for diagnosing and fixing field problems if no one in the support organization could -- and sometimes that requires the person most expert in the area work the problem for days with limited "off time" (in the old days, it occasionally required getting on a plane with two hours notice just before a weekend and staying for a week or to help with diagnosis and problem resolution -- although I tried to take those trips myself to reduce disruption to ongoing development schedules). As well, even the best planned projects sometimes get behind schedule due to design oversights and a slip will cost the company substantial money (due to contractual obligations or not being able to recognize revenue until contingencies are met) and in these cases, folks are expected to step up and contribute what they can, not what they would like to.