Something I have always found . . .perplexing, is how in "Balance of Terror" the "Enterprise" was far superior in engine design and speed yet by the time "The Enterprise Incident" rolls around the Romulan's EASILY catch-up to the "Enterprise" which is traveling at warp 9. How did their engine designs advance so far in two years?
It advanced by the Romulans simply buying the Klingons' technology. It's not like the Klingons said "Oh sure, we can enter into an agreement and you can use our ships. You just have to use your own engines on them. We'll enter into an alliance--but you can't have our engine