Practically speaking, the weight of the bicycle isn't an issue unless you carry it around. Then the issue is that the rear end has beefier stays, 9 gears and that it is rear-end heavy (at least when I compare to my 1980 Vélo Sport Alpin). For climbing, what's the difference between having a 25 lb bike and a 30 lb bike with similar geometry and bearings? Almost none. Add 40-50 lb of gear, however, and that becomes a significant issue.
The general problem with the Trek 520 gearing, however, is that it is geared like "road" bikes, with the idea that one would push hard or even stand up when climbing hills. While that work in town (providing you don't wear your Sunday's best) or on light dayrides, it doesn't work on loaded and/or long rides.
The "concensus": negociate with the bike shop to have lower gears... which is something many shop oners are willing to trade for free or at a relatively low cost as they may "recycle" what they take off your bike.
Option 1 is to keep the Shimano 105 and replace ring(s) by 52-40-24
(or 26, take the smallest). Eventually, when the cogs wear, you might replace them with Sheldon Brown's Century special (14-34 -- see http://www.sheldonbrown.com/harris/k7.html
Option 2 is to ask the shop to replace the crankset and bottom bracket with a LX or XT MTB crankset. This is what I have done and I now have 44-34-22 rings. Now that, 2 years down the road, I have replaced the cassette with a custom 12-14-15-16-17-19-21-25-32, I have gears between 18,5 and 99 gear-inches.
For a gear calculator, see http://www.sheldonbrown.com/gears
and compare with what you have on your current bike.