I have the GAP tool and have been very happy with it, though I am not using it so often, to be honest. I sometimes raise my height if I am on a longer off road journey, as the speed warning can be very annoying. I do not recommend raising your height for normal roads if you do not need to, however, especially if you traveling on gravel roads and using larger tires. I did a number on the paint on my doors and wheel arch covers, as the gravel kicks up and sprays the side of the car. This is more pronounced if you are a little over-zealous in your driving. I was more or less in a rally driver's mood over some gravel roads here and managed to get quite a few marks on my car, even though I have the standard mud flaps and the prospeed rock sliders (no steps, however).
As for tires, I have had both Cooper Discoverer AT3 and BFG KO2's on my D4 in Iceland. I had them in 265/65/18 on Compomotive wheels. Neither required any mods, and I have not noticed any issues with rubbing, other than a tiny bit at full lock in the BFG's when new. The Cooper were a minor disaster for me....I had multiple flats....at least four, though one was certainly my fault. I simply do not think they are strong enough for the D4. The BFG's on the other hand have been brilliant. They are louder and a bit harsher in the ride, but they have great traction off-road and I feel much more confident with them. Be aware that as heavier duty tires they are supposed to be run at higher pressures. I talked to BFG directly and gave them the vehicle specs and they told me to run them at 50 front and 56 psi rear, and to deflate by 7 or 8 psi off road. These pressures are extremely hard...I honestly bring them down to 30 or so for off road and feel like I could easily go lower, but just so you are aware that they are not the same as the handbook PSI ratings...
I should also say that I have one Cooper still in place as a spare, but it is a minor nightmare to get it into the spare slot while at normal pressure. Better to deflate. Mine got stuck once (not as much the tire as the winch), which of course you only find out in a situation where you need the tire. It happened to be 9pm on Iceland's version of Independence day and I still had two hour's drive home. Once I got it on the car, another tire got a flat just a few kilometers down the road (on a regular asphalt road). Needless to say, I was out of commission and I had to embarassingly ask my friend's daughter to come pick me up in her subaru station wagon. Never again...I will stick with the BFG's.
