Another vote for tires. The Compass Rene Herse models, especially the wider ones have been a game changer for me. Of course, I have to go to my loan officer every time I want to buy a set, but if you want to upgrade the performance on a bike, tires will make the biggest difference.
__________________
If someone tells you that you have enough bicycles and you don't need any more, stop talking to them. You don't need that kind of negativity in your life.