If you are going with the midlength handguard armalite carbine, you might want to think about getting a midlength Knights FF RAS. It will be the correct length. The SIR just wont be quite right.
To my way of thinkning, you are getting a flat top gun which is ideal for setting up optics. If optics are going to be your primarg sighting system, why get the Armalite midlength handguard gun. Its nice that the midlength handguards give you a slightly longer sight radius, but it sounds like you are going to be using optics. If that is the case, that longer sight radius will not be very useful to you and you'll have a gun with non standard handguards that will prevent you from getting products like the SIR... for no reason.
I'd just go with the standard carbine length gun, get the SIR you want, flip up front you want (PRI is awesome) and have a blast!