I used to be against these cheaper kits from Ractive and the like but I honestly think you'll be fine with these. Unfortunately, if anything, I feel like you may have paid too much for them simply because similar offbrand kits are pretty much the same IMO and sell for less. A friend of mine has been running a Matrix sleeve kit for about about 2 years now and he hasn't had any problems yet. I haven't heard of too many cases where these kits fail if installed properly and if you want custom spring rates, I'm willing to bet that you can use GC/Eibach or Skunk2 springs. 2.5" diameter seems to be the standard.

As long as you're happy with the purchase, and they prove to be durable in the long run, that's what matters.