I have been running a set of 245/75/16 General AT2's which was 30.5" tall, this was stock height no spacers. My gas mileage dropped significantly after the tires had been installed roughly 10.5-11mpg. I also recently discovered an issue that I created, however I fixed the issues and have been seeing roughly 14-14.5 City, about 2 mpg less than before the tires, I have not done any highway driving in the truck since the changes but I will add the numbers to one of my threads later.
While the wheel spacers are not required, they do push the wheels out more giving the truck a more aggressive look. I have pictures posted in the "What did you do to your GMT today" thread. The spacers also allow you to run tires larger than 30.5", however having the 3.42 gears (what I have as well), it is not recommended to go over 32" tires. As it has been stated the adapters will change your lug pattern to the more common 6x5.5 (just about every other 6 lug vehicle uses this pattern). It greatly expands your wheel options.
Honestly if you are going to throw in a leveling kit for the front (I believe Marks .5" upper strut spacer would level most of our trucks), you might as well get the 2.5" kit. It will level your truck as well as give it a bit more height. Its not a huge lift, but enough to know its there.