Vintage looks, contemporary performance

As this post is based around performance I would think you need to ride a high performance modern bike and an older style steel thing (even with quill) back to back to see how noticeable it is for you at your performance level and if it is noticeable whether it actually matters/is worth bothering about.
If you are are not racing on it then what is a few seconds over an hour. Can't imagine that would be noticed without a stopwatch.

You may well have done that so ignore me but thought it was worth pointing out.

I dont think speed is the real factor here.

I think you can ride a relativly noodle-like bike, just as quickly as a uber stiff one. In fact on longer rides the more flexible one might leave you feeling fresher and able to ride faster. Unless your chain stays are getting warm, all your energy is going to forward motion. It just doesnt feel fast, and the feeling is what we really want.

For me front end stiffness is massively improved by a 1 1/8" ahead stem set-up.