It does makes sense that their resting heartbeat is 300 bpm. Because earth's crustacean family also has copper-based blood (I'm a huge fan of sauteed lobster tails and I'm sorry, but Mr. Roddenberry didn't invent that concept; it already exists on this planet) but copper is only 25% as efficient as iron in delivering oxygen compounds. Now that's no problem with cold-blooded ocean life because they only burn a tiny fraction of the amount of oxygen as us warm-blooded mammals. So for copper-based blood to properly oxygenate a mammalian body like a Vulcan, the heart would have to be racing the blood around the system and lungs many times faster than a human heart, to compensate for copper's shortcomings compared to iron. It's probably the same reason why whales evolved the ability to hold their breath a long time and sometimes surface to breathe - as opposed to evolving gills like fish; because whales are mammals and fish gills could never provide the amount of oxygen that we endothermic life require. I'm a science/medicine geek who's worked a number of medical-bureaucrat jobs over the years so these are naturally the kinds of thoughts that occur to me.