One reason I try to go to outdoor fittings by Titleist or at Premier fitting sites is they have a doppler radar unit (i.e. Trackman) and a whole pile of different shafts. As experts, they can find a shaft and head geared to optimal performance for my swing. Guessing on a new shaft for $80-300 that is not optimal is far more expensive than a fitting.

I would find a Titleist Thursday near you or a professional fitter with the ability to fit outdoors. Titleist Thursday's do not cost & you have so many selections to end up with the right shaft & driver head. I have the same shaft in both my 915D2 & 915F, but my 816H1's have a different shaft but I was fitted by a Titleist Fitter so I know they fit me & my swing.

Note:
If I didn't get fitted I would still be using stiff shaft clubs but my fitting told a different story

agree with Don O

In my opinion, for whatever that is worth, the shaft is one of the most important components of the club. Having the correct shaft to fit your swing profile, matched to the correct head, is what fitting is all about. When you work with a fitter and try all of the different shafts that are available, you can see first hand how they can impact your shots. Get with an authorized fitter and you won’t be sorry you did.

The technology of golf equipment has developed to the point where a professional fitting is a requirement to get the most out it. The head on a driver is manufactured to specifications that must meet USGA standards. The rebound on the face (COR) has to be within USGA standards and the clubhead size also. Most manufacturers driver heads push to the limits on COR and size. The defining component of a driver now has become the shaft. A properly fitted shaft can be the difference between "not being able to hit it consistently or for any distance", or "striping it long and straight consistently". I can't emphasize this enough, get professionally fit for your driver !!!

Look at this way. The staft is the "engine for the golf club". Having the correct shaft is very, very important!! Good luck.