Given your level of fitness, I'll agree with a better wheelset and keep the front tire the same width as the rim. Try tubies for the lightest wheelset - doesn't have to be carbon. I also agree with ShootinCoach that the whole package is the best improvement. And intervals from time to time for power.

But my 'hidden' secret is trying to keep myself as aero as possible, even at 15, 16, 17mph and at all times. No sitting up in the wind. Everybody tries to keep a flat back, but that may not be your power position (at the group's speed+distance). Hands closer to the stem, round your shoulders, elbows tucked in, pedaling with knees almost brushing the TT. At some point in the future, ditch the wide handlebars. Even read somewhere that shaving the legs helps (maybe nonsense, what do I know?). If you are a multi-bike guy then get one set up for speed. edit: dump the helmet and shave your head if you are really serious about this, or perhaps a $250 new aero road helmet.

Just remember all these improvements are minimal. Assuming no over-riding medical conditions - nutrition, exercise and rest in the proper proportions are the key. Years and decades of base miles also train the body to sometimes go beyond. Or maybe it's just the Voigt Principle at work.

And of course - drafting 'as-close-to-the-wheel-in-front-as-you-both-dare' can be the most significant. But generally frowned upon in amateur group rides, where no one expects to go down.

Sounds like you are enjoying it - so don't pass up the chance to enjoy the scenery along the way.