Your best best is to get fitted from a professional club fitter.  They will tell you the best shaft for your swing.  There are plenty of good shafts with stiffer flexes, but you want to make sure that you get the one that is best for you.

I have S300s and love them! Make sure you really need a stiff shaft though. 

I will not pretend to be an expert, but as a general rule, the shaft choice must be based on your game, what you want to do with the ball, and what you have a hard time doing with the ball.  The AP1/AP2 irons are built to help players hit higher golf shots, consistently with some degree of off center strikes.  (I know that is an over simplification)  Perimeter weighting provides a larger sweet spot, and the weight oriented to the bottom of the club promotes higher launch and more spin.  When the AP irons are paired with a Project X shaft, it is again done to promote higher launch, with a flat trajectory .

The stiffness of the shaft impacts the amount of spin that is applied to the ball.  If you were hitting big high hooks or slices, or have a lot of lateral movement in your shots, a stiffer shaft may help reduce spin rates and help you hit straighter shots.  If your issue is that you hit the ball very straight but lack distance, stiffer shafts probably wont help using the same swing.  If you hit the ball straight but too low, than moving to a project X shaft may improve your ability to hit the ball higher.  

The basics of 2 popular shafts are as follows

If you want low launch and less spin, try dynamic gold (R/S/X)

A softer version of DG (launches a little higher, spins a little more) is the KBS shafts

If you want higher launch and more spin, try Project X (4/5/6)

If you feel like you are hitting the ball too high, and you are using Dynamic Gold shafts, moving away from the AP's to a CB or an MB may help you keep the ball down.  Moving to a project X shaft will push the ball higher, not lower, and may not help you achieve your goal.

If you like your ball flight and spin rate, but feel like you would like to hit it farther with the same swing, a softer shaft may be better, if you want to hit it farther with a harder swing (but maintain your accuracy), maybe a stiffer shaft will help.  If you have inconsistent distance shot to shot with the same club, maybe a different club head (MB or CB) will improve your shot to shot consistency, but will take away some foregiveness.

So, the answer to your question is a question. What is your current shot shape through the bag (meaning with all your irons)?  What do you feel you do well, and what do you want to do better?  Answer those questions, and then you can start to figure out which shafts promote that result.

Hope that helps, and good luck...

Just so you dont think I am a hacker with no clue, my current HC is 0.3, I play - 710MB's with X100 shafts. My problem (primarily) is I hit the ball too high with a too much spin...that setup helps me hit it where I am looking, and control the ball movement, with minor reduction in distance.

I agree.  I was always a DG fan until I just got the Nippons.  They are a very good feeling shaft.