NYS law prohibits pistols over the formerly Federal 50 oz limit. Bushy Carbon-15 pistols are legal in NYS, adding a normal upper would put this over the weight limit.
Between NYS and NYC law, there can be no legal proposed use for an 11.5" upper in NYC unless it was for a handgun registered prior to 09/94.
Unless you have a pre-94 registered pistol lower in NYS, I'd avoid playing with something like this.
NYS has no provision for NFA SBR's or the like.
Check with the NYS Rifle & Pistol Assn. http://www.nysrpa.org/ , they have all of this on the website. http://www.ci.nyc.ny.us/html/nypd/html/dclm/ldinfo.html is the NYC link.
Under NYC law the following is prohibited:
(c) Any part, or combination of parts, designed or redesigned or intended to readily convert a rifle or shotgun into an assault weapon.
An 11.5" upper would fall under that description based on the text in the law, even if the item were being redirected out of the jurisdiction. Better to receive it elswhere if it is for use outside of the Empire State.