Flexes should stay consistent throughout your set, because your swing speeds and tempos, for the most part, are usually the same with all of your clubs.  Some players may tend to have faster tempos with their drivers, which if that is the case, heavier weighted regular flexes can help counter act this.  You may be seeing better results with the stiff shaft in your driver, because in some cases the stiffer shafts can help with dispersion.  Your swing speed could fit into a regular or stiff depending upon your tempo, which seems to also be the reason for your success.  You could try switching to the Diamana, but if you start to hitting more inconsistent shots, then the flex may be too light. 

Cameron

1-888 TITLEIST

Cameron,

That's for the great reply.  Very informative.

Just one question, when you say flexes should stay consistent throughout your set, are you referring to irons too?  I play DG stiff shafts in my irons and I really like them.  Would I see better results with a regular flex in the irons?

Yes, I was referring to your irons, but knowing your driver club head speed, you can reasonably assume you have a relative club head speed throughout your set.  With that in mind, you may be able to see accurate results with your DG S300.  Before making any of these changes I would recommend seeing a certified Titleist Fitter.  They may be able to show you that your iron ball flight is consistent, but not achieving your true potential of what you could get from a playable trajectory or spin.  

If you wanted to get fit, here are a few locations around you that would be reputable options: 

Windyke Country Club

8535 Winchester
Memphis, TN 38125-8228
901-754-7273

Edwin Watts Golf Cv55

704 N Germantown Pkwy
Cordova, TN 38018-2252
901-757-1886

Golfsmith #067

7741 Winchester Rd
Memphis, TN 38125-2304
901-755-1504

Hope this helps! 

Cameron

1-888-TITLEIST