It's the size of the tire that makes the biggest difference. Weight matters but nowhere near as much. The taller tire is basically regearing you with a lower gear ratio. Basically what it will mean for you is kiss 6th gear goodbye. Probably 5th as well. They will only be overdrive, absolutely 0 pull left in them. It will be similar to you 35s on 3.21 gearing.
This ^^^
Weight is the least of your concerns. A bigger tire is essentially a gear and changing this WILL change the performance of your Jeep - make it more sluggish. Also, the tire sizes you see on manufacturers websites are a joke. The side they show are what you would see on a tire fully inflated, measured to the furthest edge and without the weight of a vehicle on it. The following link will give you a better idea of what real tire sizes come up to:
http://wayalife.com/showthread.php?35075-The-Ultimate-TRUE-TIRE-SIZE-Database