I've always used "picatinny rail extenders" when more rail-length was needed..
They work great, but, they do add a bit of height since they're pretty much just add-ons.
I've seen these K&L uppers, and think that they're a great idea to maintain the 'oem look',
as well as the fact that they're 1-pc construction, so, less chance of misalignment issues..
I use K&L products, and recently had a tech question for them..
Not only did I get a lightning fast reply back, I received better info that I could have expected,
and then a few follow ups for a side conversation while just talking shop..
GREAT company and products! And Highly Recommended!
Sam -