I think the only true upgrade on my bike was the switch from a Tiagra front derailleur to an Ultegra. But that was not planned and I didn't pay for it -- I brought the bike to the shop for an FD adjustment, and it broke while the mechanic was working on it. It was probably metal fatigue from the million or so previous adjustments that were necessary on that FD, but the guy felt responsible so he replaced it at no cost, and the only triple FD they had in stock was an Ultegra so that's what I got. It was a huge improvement. The Tiagra was always causing problems with dropped chains, but I've rarely had that problem since getting the Ultegra.

I tried some Vittoria Open Corsa tires a couple years back. Before that I had ridden whatever Bontragers came on the bike and another set of Bontragers that were supposed to be puncture-resistant but really were not. I liked the Vittorias a lot for their decreased rolling resistance, but they got too many punctures so I switched to Continental Ultra Gatorskins which I love. I'm more interested in avoiding flats than having race-quality tires.