I am inclined to agree with Ken....same gear for an overnight as a thruhike..at least that is the train of thought my wife and I had as we have updated a lot of our gear this summer. Lots of time in EMS and other stores till we just honed in on the online research and talking to some long distance folks.
We changed the following:
Was Nalgene bottles/bladders -> Now Gatorade bottles
Was Water Pump - > Now Aquafina chem
Was Cookpot -> Now MSR Titan Mug (we just boil and rehydrate so its plenty big)
Was More clothes carried -> Now far less clothes carried..but smarter clothes
Was Kelty 7 lb tent -> Now Six Moon Design Lunar Duo...just over two pounds
Was Slumberjack heavy and bulky - > Now Montbello Spiral 35s...just over a pound
But the biggest single change was in the pack. She had a Northface pack that was big and heavy...I had some no name sams club thing I had bought in 94.. We both ended up getting the ULA Circuit, and are very pleased with it. It is more than large enough.
There is others items probably, but the tent, pack, sleepbag made the biggest difference. It may come down more as we upgrade some clothing to lighter weight gear as well.
Base weight for the lady is 12-14lbs
Base weight for me is right around 14-15lbs
Actually, we started looking on white blaze and other sites for gear thoughts on gear thinking thru hikers would have the gear dialed in - which most do. The research was for just doing more backpacking here in New England (we live in the White Mtns)...but after a lot of reading, we are now planning to do the AT next summer..