I have installed 4 or 5 of the Daystar poly mount lifts. Both of our YJ's have them, and I installed the others in YJ's/TJ's/LJ's for other people as well. There is no sign of any sagging (or other issues) in any of the kits I've installed. The only recommendation I have is that you use the black mounts. I have the "red" mounts in my '91 YJ and the rear corner ones have faded to an orange color. This appears to be limited to that ones that are colored, as all the rest are black kits & they still look like new.
From all I've heard, the Prothane kit is pretty much the same kit, and works equally well.
Here is some advice for installing your new body lift, regardless of which brand you choose.
Below is a "cut & paste" copy of a reply I've made in a few posts, concerning the body lift installation you're considering.
Regardless of whether you are installing a body lift, or just changing out the cruddy stock mounts that are broken, here is something that may help. It's something I've learned over the years, the hard way, naturally.
I've installed a half dozen or so body lifts on YJ's & TJ's. I found a simple little trick using the PB Blaster that will save a lot of time & aggravation. Spraying just the heads of the bolts won't help. If you look at the 11 body mounts, there is one under the radiator support and then 10 (5 along each side) on the frame rails, starting behind the front wheels. The forward-most mounts can be sprayed near the threads by removing a plastic shield & spraying the nut area. Moving rearward, the next three body bolt nuts are in-accessible, under the floorboards. Inside the Jeep, locate the area of the floor right above each body mount & drill a tiny hole in the floor just big enough to get the spray can straw in, and spray generously. Lastly, the ones in the rear corners of the tub can be accessed by removing the plastic shield under the body's rear corner. Look above the mount and you'll see that the body nut area is boxed in with sheet metal. Again, drill a tiny hole & spray liberally.
Do this daily for about a week prior to the body lift install, and things will go much
smoother! The holes can be filled with just a small bit of RTV sealer later.
I learned this the hard way, after
breaking several body bolts in my '91 YJ.
If you have any reservations about drilling small holes in your floor, here is how big
of a hole you'll need to dig out the body nut after the bolt breaks!
Here are some pics of how I cut mine out, using a 2" hole saw. I dug the captive body nuts out, and then used washers & nuts on the body bolts.
In front of driver's seat
Behind driver's seat
Hope this helps!