The 6" springs are stiffer spring rate than any of the OME springs out there. We developed the springs to 1) reduce costs in the lift kits since we do not have to machine spacers and 2) the OME springs with spacers on them in certain cases was not enough spring to control heavy loaded vehicles and 3) to get a system that sit closer to level at that height.
Stiffer springs off-road will have a little negative effect as well since they will not be as soft and allow the suspension to flex as easily. However for people that wanted tall lifts and excellent road manners needed stiffer springs. As NW mentioned, the OME's with spacers will be softer and give you a little edge off-road, but it is all a compromise at this point.
As for large tires (36" and up) there is a point where more lift only hurts you. With 6" we are already using all the control arms at much bigger than stock angles (especially the rear) and anything above that would require custom setups.
Merely lifting the truck with 6" springs and spacers will not allow you to run 38". You are going to have to trim some flares and/or sheetmetal. The problem comes in with compression (which effectively makes a 6" lift 0" lift) and the wheelwells are just not big enough to accomodate the tires. A truck with 6" lift is already big, adding spacers to that is not for the faint of heart and you really enter an area of experimentation and testing to see what you like.
There is also the issue of control arm lengths and the relative movement of the axles due to the "shortening effect" of the lift. I am sure NW will experiment with this and determine the optimum setup for him. The statement that the lift will clear 37's is true, however it does not mean that one does not have to ramp and trail test the truck and adjust bumpstops etc etc to make that there is enough clearance. Also, tire selection and wheel selection plays a big roll. We advise all customers that do the 6" lift to get wheels with less backspacing or run wheel spacers. Also, in a lot of cases a 10" wheel is a better choice, but that will open up another set of rubbing issues.
Hope this helps.