One way you can do this is to create a Table report, add the relevant metrics you are interested in, such as Delivery rate, Total opens & Open rate. Then under "Group by", select "Subject".

This will give you a row per subject, so you can compare performance.

However, this will not be as useful when you have personalised your subject lines (for example, including the contact's first name) as this will result in a unique subject line per person, not grouped by the raw subject line before the merge tag was replaced.

This should help for some use cases, but I will leave this ticket open until we have a good solution for the case of using a personalised subject line.