I also have just fitted Dunlop GT501's replacing the stock Metzelers on my 2008 T100. It's a little too early yet to make a judgement on their performance as I have only had them on for a couple of hundred K's or so and for the last 100 K's of that I was riding two up which means I was riding much more conservatively than I usually would. Basically I'm yet to stretch their legs and find out just how good (or bad) they are.
I had a little over 16,000Klm's on the Metzelers before they were replaced and in the last 2,000Klm's of their life their performance had dropped off considerably, the rear was prone to slipping in curves if pushed just a little and the front was cupped enough to cause front end wobbles so the Dunlops "appear" to be far superior when you step off worn Metzelers straight on to the new Dunlops.
Not sure if it was my lack of confidence because of the state the old Metzelers were in or my caution regarding the "slipperiness" of brand new tyres but it did take some time to adjust to the feel of the Dunlops, particularly the slight increase in height of the rear due to the size increase.
To sum up, I was figuring that I was "adjusting" to the tyres but there may be some truth in the initial "heaviness" story regarding the GT501's because mine certainly "feel" better after a couple of hundred K's than they did when first fitted.