A fascinating design, beautifully rendered!
I'm not convinced by the idea that Star Fleet developed this ship and the Romulans copied it. Apart from anything else, Romulan Birds Of Prey of a similar design appear in ENT, in 2152. Maybe this was developed after 'Balance Of Terror'. Trying to invent their own cloaking systems, Star Fleet experiments with ships based on the Romulan design, in case that particular shape is more efficient.
Even without a working cloak, the design is effective and put into production.