I would think that whatever your normal flex is, based upon your swing speed, would suffice. If you go firmer based solely upon loft, you might be compromising whatever gains you would otherwise expect from the higher lofted driver.
I added an inch to my 11.5º driver shaft. Kept it at regular flex.. Trajectory was good. Would rather see you go lighter versus firmer, unless you start hooking your tee shots. If you're hooking your tee shots, then you might be better off going firmer verses lighter.