Nothing at all wrong with a 28" 2315 at 70 pounds. It's an 'A' stiffness on the charts all the way to a 5" overdraw.

I definitely agree with FLHunter about the possibility of a false tune. I don't know what cam system your bow has on it. PSE doesn't list a Predator model on their site (might be a proprietary name for some pro shop or mail order outlet), so I can't see it. Anyway, some bows can have a sweet spot close to the riser where an arrow will tune to bullet holes but be so far out of centershot that you can't get decent arrow flight or hit anything with it. With a stiff arrow, it's not too hard to get way over there.

Put an arrow on the string, hold the bow out in front of you like you're going to draw it. Eyeball the string to center in the cam groove, then look down the arrow. Is the tip of the arrow pretty much lined up with the string or is it way off toward the riser? If it's off toward the riser, move your rest to line up the tip and retune.

I'm not to hot on paper tuning anyway and really not to hot on paper tuning with a bare shaft. Have you shot the unfletched arrow at about 10 yards and just watched how it hits the target? What direction does the nock end wind up in relation to the point? Does that bare shaft group with your fletched arrows or is it well off to one side or the other?

If what I'm thinking has happened, I would expect the bare shaft to strike the target with a hard tail left angle, assuming you're a right handed shooter.