Isn't the spine stiffness based on the arrows "original" length of 30" or 32"?

I think you'd have to figure in arrow length too. If he has a short draw length he could cut the 4555's down to the "correct" length and stiffen the spine that way.

However, if he has a longer draw length 29 or above, I'd say the 5575's would be better.

my 2 cents