When I had my RK 2.5 Max Travel installed, I got 4" front, 2.5" rear. I have factory bumpers.
Note: I had a JKU Sport, so had shorter springs compared to a JKU Rubi.
RK springs run tall. Everyone I talked to about getting this lift recommended replacing the front driveshaft. I did replace it when the lift was installed.
3.5" would be suggested if you want to run 35" tires or larger.
I do also recommend either relocating the evap, or moving the skid (2012+) to the left a bit. Shortly after getting my lift, while wheeling, the rear driveshaft contacted the evap skid and tore the boot. I've been driving with it torn for about 9 months and no issues. I would recommend preventing this from happening though.