I ordered a RRA  A4 mid length upper and a Troy buis today.  I am wanting to know if I need to get a taller front sight post or will the one from the factory work?  I plan on using the Troy as my primary sight for a while.  

In truth you won't KNOW till the rear sight gets there and you zero the carbine.  If the tolorances stack in your favor you won't need it.  If they don't stack in your favor then you will need it.  The only way to tell is to go to the range and try it out.

Edited to add:  I keep a spare tall front sight post in my parts box - just in case I build a carbine where they don't stack in my favor.  For the cost of one I'd recommend you do the same (and make sure you have the front sight adjustment tool as it makes installation much easier).

Good idea, for $5 there is no reason not to have one on hand.  It would be nice if bushmaster would drop it in an envelope in stead of having to pay $6.50 for UPS.