I have a buggy with 12.5" front and 6" rear, another with 12" front and 7" rear and one with 15" front and 14" rear. All have had their shocks dialed in over time. I don't feel the 15" in front one has an advantage over the 12" one as far as front travel. I have not bottomed the rear of the shorter travel rear cars much, but I think 12" there would be nice there. I would not raise the ride height much with more travel on them. The 6" travel rear car only has 1" droop. with 12" of travel, I would set it up with 50% droop, so 1" higher ride height and 6" droop travel.
This is not to say I would not build a car with 3' of travel just to do it, but I would not think it was practical while doing it.