The first thing you should know is that regardless of how it is advertised, the likelihood of buying genuine FN parts is probably about 1 in 10,000. At most. Lots of people and dealers advertise their no-name or less-than-desireable name parts as FN.
Myself? I've kept all of my Colt AR15s 100% all Colt. I like the assurance of knowing that I am getting a product that meets Colts quality standards instead of something hacked out in who-knows-whose garage.
All AR15 barrel extensions have the two feed ramp cuts. What is unique to Colt M4 barrel extensions is that these feed ramps cuts are enlarged and there are corresponding cuts in the upper receiver so that the feed ramps actually extend down farther than typical to aid in feeding ammunition. With a USGI barrel assembly, you can use an M4 upper receiver but will gain no benefit from doing so. You could also check the Equipment Exchange for a used Colt non-M4 flat top upper receiver upon which to build your project and maybe save a few bucks over an M4 type.
With a genuine Colt bolt carrier assembly, you know that you are geting a quality product. The gas key will be properly staked on and the carrier and bolt itself with have been magnetic particle inspected to ensure there are no internal cracks or flaws.