This is one of the things that just totally annoys me about "fannish" ideas about starship design. The EXTERIOR SHAPE isn't what makes a ship what it is. You could have two ships of identical shape which would be totally unrelated in every meaningful way.
This is why I've always held that the "refit Enterprise" was a different class of ship. In every meaningful way except what it LOOKED like, it was different. It had a different power output, a different top speed, a different cargo capacity, a different sensor suite, different weaponry... and when you get right down to it, it wasn't even the same shape, just a similar shape.
So, let's suppose that the Romulans saw the exterior configuration of a Federation starship. Would that let them replicate the TECHNOLOGY within that ship in any way? Probably not.
There are issues in "fanon" having to do with ship configurations being more, or less, "warp dynamic" (think "subspace aerodyanamics" and you'll follow). And if you look at modern MiG fighters and modern US fighters, they often look quite similar. But does that mean that one is the COPY of the other, or that they happened to be developed in-parallel (using similar approaches, perhaps) and the actual laws of physics guided them towards similar configurations? I tend to think the latter, rather than the former... because I've seen REALITY guide separate design teams to nearly identical solutions many times. And every single time, the people outside of the engineering teams always assume that one of the teams "had to have copied off of the other team."
Of course, there's also the fact that the Romulan BOP doesn't look all THAT much like the Enterprise... certainly not in any way which would be representative of "subspace aerodynamics."
If you wanted to draw parallels between the BOP and Federationships, you could, though. You might notice that the exact same hull construction was used (some combination of laminated plating with some combination of coatings on the outside, maybe?) which would look awfully familiar. You might notice the power generation of the other ship would be operating on the exact same frequency with the exact same set of harmonics. You might notice the impulse thrust system was putting out the same set of particles in the same ratio, and at the same energy levels (ie, the "exhaust is the same temperature and smells the same").
Oh, and you might notice that the FTL propulsion system nacelles are constructed similarly to, and output energy and heat in the same way, as your own.
It's not enough to be proof, but it might feel a bit coincidental.
The point I'm making is that the "look of the ship" isn't what you'd care about. You'd care about the individual bits and pieces, the materials, the detectable technologies. The "ship shape" would be the least significant thing to look at.
The TMP Enterprise couldn't have been made by "copying the shape" of the TOS one... and it wasn't in any meaningful way the original ship (from construction to technology to capabilities, everything was different).
"Same shape" isn't an indication of copying.
"Same technology" would be.
That's my point.
