What she said -- I got the 4.5 due to budget, so I asked a lot of questions about upgrades over time.

The one thing you can't upgrade is the frame. My understanding is that with each whole number in the Madone series, the frame weight goes down by about a pound. So yours would be a pound lighter than mine, and the 6 series would be a pound ligher than that.

I hate Bontrager saddles, so I had the shop swap that out immediately.

The number one upgrade the shop guy recommended, if I felt like it, was a lighter wheelset. That difference may be noticable, whereas shaving a few grams here and there by buying higher end components probably won't be.