PH, I noticed the weight on the first day (I've never used 1st and 2nd gear before), less on the second and not at all on the third. I had to remember not to 'honk' out of the saddle though.
The only problem I had carrying all the gear at the back was up very steep hills in 1st and 2nd gear. It had a tendancy to wheelie which lo-riders at the front would, I suspect, cure.
Overall though it didn't handle much differently and I had no issues with fast descents. I was carrying about 12 kilos. Small tent, sleeping bag, sleep mat, stove, pans, clothing, shoes, wash gear and so on. It was only a 3 days tour but I doubt if I'd need much more to do a longer tour The Ortlieb panniers are superb, really pleased with them. note, my buddy on the Blue Raven (pictures) has 'upgraded' to ortlieb since he saw them in use on the tour.