Shaft flex .

If you were properly fit for a regular shaft then use a stiff shaft you will not get the shaft to load properly in your swing. This will lead to low trajectory shots which don't carry very far. This could also lead to slice as well as the club head will not properly return at impact leaving the face open.

I would agree with Michael. IMHO the most flexible shaft you can control is the best for your swing.

Swing speed and type of swing (99% of the time)

Why would you use a stiff flex if your fitter put you in a regular? That's like getting fit for a suit and seeing a different size.
I would think you would was a loss of distance and a lot of balls to the right. Slices push ext.

I got fit for the project X 5.0 lz hard stepped once which is considered regular for my 716 ap2's!! Come to find out when they ordered them, Titleist doesn't offer the 5.0, but the 5.5, so we soft stepped them twice!!

I have a disintegrating L5 disc in my lower back so I don't swing as hard as I used to!! I hope it works out!! I am a Titleist guy, but have hit miz irons for 8 years and they offered the 5.0 custom fit!! Thought of reshafting my MP64's, but went with the ap2's!! I hope I don't regret it, the 917's are awesome!!

90% of all golfers buying clubs off the rack have less than an optimal fit. I know how bad I am as my fitter . That made it easy for my fitter to put 20 more yards on driver into play. Only part of it was the 917 head. The right shaft recommended was the biggest part.

The ball will be going right, one way or another.....

Agree the game is hard enough to make it harder and have the clubs work against you. If the project x shafts aren't ideal or available for your swing, I'd definitely recommend going back to the facility where you were fit - I'm sure they would refit you to something available and stock from titleist. They use premium shafts and tests them all for compatibility with their clubs. Good luck the ap irons are fantastic. Enjoy

I am in the middle of trying to figure out what is best for me. Currently play XP90 R300 in both the 716 AP1 & AP2 irons. I am considering moving to XP95 S300 or XP105 R300. I am not getting any younger (almost 59). Play the Diamond Blue S+ 72g in Hybrids (stiff) and Blue 71g LTD in Fairway Woods (stiff), Blue 64g LTD in Driver (stiff and a regular (currently testing) shaft ). Driver swing speed in low to mid 90's. Irons Swing Speed mid to high 70's. Work in progress. I feel the XP90 shaft might be a little to whippy for me. We'll see. Good Luck !!