I am lusting after a RRA M4-style upper. ADCO sells them with an option to cut off 1.25" of barrel, recrowning, and adding the 1.25" long Wilson brake to get back to 16". This is in comparison of slapping the 1.25" long brake in on top of the 16" barrel. I really like the looks of the shortened barrel, but I'm wondering how the shorter barrel will affect reliability. It seems like the amount of pressure going back through the gas tube will be different, albeit slightly. Is it too minor to affect functioning ? Anyone with one of these who could shed some light on this ?

Here's a picture:

A little bit backwards. The carbine gas system is designed around the 14.5" barrels, but they work fine with the 16". Shortening a 16" will not be an issue.

Actually the 14.5" was designed around the existing carbine gas system, but whatever. 14.5" barrels work well for the military and a lot of civilians so clearly reducing a a 16" barrel back to 15" shouldn't cause any problems.