I have this kit. I don't know what the add claims that you are reading however the one I bought was supposed to be a 2.5"f and 3/4" rear spacer, plus the 125" body lift. For starters they sent me out 1.75" rear spacer, so the truck still had a massive rake and just generally looked stupid. I called them up and they basically just told me it was a production change, tough luck. I never did get what I ordered. The supposed 2.5" front spacer actually achieves just under 2" of actual lift, so that also helps to keep the massive rake when paired with the 1.75" rear spacer. I have no complaints about the body lift except that it tore my rad shroud all up when the fan turned on, it could be a bit my fault for not thinking to check, but still, I thing they should have mentioned to check it in the instructions. The worst part for Rough COuntry is that I was planning on buying their 6" lift this winter, however because of this whole ordeal I am instead going through bds. I'd hate to spend big money on a lift and have them tell me tough **** again when something's messed up.
Final Recomendation: If I were you I'd just buy the rough country body lift and the bilstein 5100 front shocks. This will give you a total of about 4" of lift for just over $300, which isn't a bad price at all. Even better is that if you decide to go with a suspension lift in the future, you can still use those parts in conjunction with it, so you aren't wasting any money by buying this now.
You also asked if it was noticeable. Well, I also have 35's on my truck, and in person, people ask if I have a lift in it, and other people say it's a big or stout truck, however IMO the 35's look too stuffed. Realistically, if you aren't looking to go huge, just a bit bigger, a 6/3 lift is probably going to be your best look, but it's also around 600% more than the cost of this 4" setup. I guess it all comes down to how good you are at compromising.; I know my new lift is coming next month and my RC levelling kit is gong in the garbage.... Compromise blows