What would keep you from having a heavier arrow with a higher FOC? Run a strong enough spine and add weight up front? ((I am asking this question, not trying to be a wise guy. I've spoke with several local shops, and the answer they give is usually "you don't need that", but I think it pertains more to selling items they have in stock)
The issue is that you will end up with an arrow that is a lot heavier than most guys want to run, so you give something up in the trajectory department. As an example, the GT Kinetic .200 spine arrow is 11.6 gpi. At 28" with standard components, you would need an ~180 gr. BH to achieve FOC of 15%, which results in an all-in arrow weight of ~560 grs. Based on what I have seen anecdotally, most guys aim for a finished arrow weight in the 400-450 gr. range.
I think both ChadS and the guys at your local shops are giving you sound advice about not trying to build an arrow around high FOC - you absolutely don't need that. IMO most guys are pursuing high FOC arrows to combat a problem with penetration that they don't actually have - and some are creating problems they didn't previously have by using light and relatively fragile shafts to get a high FOC arrow with a finished weight in the range I referenced above.
Just to give you some context, the high FOC concept comes from Ashby's work (Natal Study & updates), and he shoots a 750+ grain arrow in order to get the shaft durability plus the high EFOC that he desires (as an aside, he also shoots a long bow, which are inefficient with lower grains/#draw weight arrows). That simply isn't necessary for North American game.