How to measure success of email templates?

There isn't a template-centric report AFAIK. The notion of "email performance" is  linked to the email asset/subject/content itself.

Might be interesting to see this comparison, but you'd have to generate it in part via the API (e.g. get the Email → Template relationship via API and join that with the canned EP report in Excel).

I'd venture that most of the time, template performance comparison would relate to sending the same type of content to the same audience using a newly designed template... so effectively the same as email performance comparison. Though I can understand other cases.