regarding the weight increase, my understanding (and please correct me if i'm wrong) is that the 10-15 lb higher weight of the 2011 X is due to a number of changes over the 2008/2009 models, including:
- non-ported track
- e-tec injection
- larger dual intake system (my airbox weighs 7 lbs!)
- stronger frame
regarding the suspension itself, i'm not sure how heavy your 2008 setup is but the 2011 setup is pretty light. the upper a-arm with heim joint weighs 2 lbs and the lower with heim joint weighs 3 lbs and the shock with spring and bushings only weighs 3.5 lbs. so that's 17lbs for all 4 front a-arms with heim joints, 2 shocks, and 2 springs. since the a-arms and shocks are shorter, i'd bet the front end is the same or lighter weight than your 2008 setup but i haven't confirmed that. the rear parts are quite similar, my understanding is that the changes are due to geometry and suspension rates. i can't comment on the difference in weight between the rear setups but i'd bet they are very close (most of the weight is in the springs and arms and these are virtually identical parts)
regarding compatibility with your 2008 sled, i just confirmed with a dealer that the 2011 front end is a direct bolt on to any XP. there are a few minor tweaks that you'll have to make to the rear to make it perfect including a longer limiter strap ($15 part or mod your existing one) and a different rocker for the rear arm (pn: 503191209 - $50 part). you may or maynot also need the aftermarket "chips" that fit in the cutouts of your runners to move the rear axle/pivot forward slightly (like the tom's kit).
hope this helps.