The risk of hiding comments?
Rarely are there comments which on their own significantly add to a post. The entirety of the comments may add content, but usually only one will shine through as the comment which caused the post to become more significant or contextual.
Hiding comments is an easy one click fix for a user, so the real risk is that a high value comment hidden would have been overlooked because the user did not know it was there.
As a user looking for value in posts, comments are often a place to look when examining the critique of a post. If there are no critiques or suggestions of value, then there is no point in examining them so I think that as long as enough are shown to entice a user to click through for the entire set then that will be enough to allow users to find the high value comments if they are hidden.
Measuring the risk
When being thorough, a user may want to see every comment regardless of their quality. I don't think many posts actually require the entire set of comments to have the answer be relevant.
Split test
Test for the amount of interaction done with the comments where finding them by clicking is undesirable, and upvoting them is desirable.
I think a measurement could be in the form of click through comparisons plus upvotes. Group A sees the status quo, B sees the new version (whichever version that is). TotalCompare total points from how many times each group A triggers the reputation hoverhover for the username of a comment (+1 per username), upvotes a comment(+10), expands all comments(-5).
Whichever group has the most points indicates the most positive interaction with the comments.