In that example of +280 hp is the max, the highest amount that materia could give was +28 hp. The whole thing was an example so I'm not sure if people actually look at it that way. Right now the most a materia can give you for hp is +70. If it follows the same rules, should it not be +700 hp is the max? So 10 perfect tier IV Bloodthirst materia would logically be the max you would benefit from. This is speculation, but it would seem logical to me.
