Don't misunderstand, as I said, I wasn't trying to sound critical of your 300 miles per month, not at all. My point was that, relatively speaking, it wasn't really a lot of training, for someone working up to hanging with a group running at 20 mph. You just gotta keep on, keepin' on !.
And, yeah, as I said above, I wouldn't be surprised to see
some improvement with a nicer bike, just that I would expect it to be fairly minimal (assuming the original bike was decent with no flaws holding you back), and not really proportional to the extra couple thousand bucks. ( "the point of diminishing returns" is the phrase that springs to mind here).
Naturally, pedalling less weight around will make a difference, especially the rotating weight in the wheelset. If money is no object, then hey, you get you a new bike !!! No harm, no foul, more power to you !
But if it is, then you might consider just getting a nicer, lighter wheelset, and tires (as mentioned above,IIRC). That's gonna make the most noticeable difference, versus better shifting or shaving a pound or so on the seatpost and stuff. But be conscious of weight limits with lighter wheelsets. (which is also something you want to be aware of even when buying a new bike.)
Even with nicer Ultegra-equipped bikes, you can usually expect some parts skimping, and the wheels are about their favorite place to do this. They love to slap on some generic OEM wheels, with low-count aero-bladed spokes, that are intended to
look like what the pros race up the Pyrenees on, but ain't, and anyone weighing over 190 is at risk of busting spokes on these wheelsets (in my experience).
This is why I've often just replaced OEM wheesets, right off the bat (my current 3 bikes all have very solid wheelsets from BicycleWheelWarehouse).
Anyway, that's just some more rambling thoughts, hope it's somehow useful......