I had that same bike as my 'first' and it served me well for 3.5 years. It has been replaced with the two 105-level Bianchi's in my sig. I nabbed them both on craigslist for about half the MSRP and sold the Trek 1000 for about the same rate.

I did some upgrading on my 1000. Maybe a little more than it deserved in hindsight. It's a really heavy frame, fork, headset, and stem.

I put on used silver 105 brake calipers and new Kool Stop pads. Huge difference in stopping power.

Wheels are a good idea as (you mentioned) they are easily transfered to another bike and can take some weight off that Trek. But I only went down that road when I started braking spokes.

I also ended up replacing both derailluers (one 'upgrade', one 'cause it broke). I had a 105 RD and a Tiagra FD. Not sure that made a significant difference over a decent tune-up, but the 105 RD was noticably more responsive (over Sora).

I'd also suggest that some other gear might make more of a difference than the bike. For example, are you riding with clipless pedals/shoes?