I was so blown away that I had to post something on here about this. The other day I played with my dad's Ping G20 "soft regular" flex driver and was crushing it straight and long, between 260-70 with nice high ball flight.
I have been through a bunch of drivers in the past, none of which I have been very happy with. I usually go for a stiff, low spin shaft because that is what I see people with higher swing speeds playing. Most recently, I have played a r7 quad with a Fujikura X-Stiff fit on, and a TL310 with a NV 75 stiff, both 10.5 degrees loft. I hit both setups around 240-50, with a pretty low ball flight.
After doing some research, I found out that the effective loft at the point of contact has to be fairly high to maximize distance, around 13-16 degrees at contact, or even higher depending on swing speed and other variables. To satisfy my curiosity, I went to a local golf store and hit some shots on a ball flight simulator. It said my effective loft at contact was consistently around 11.2 degrees with a 10.5 degree loft driver head. Therefore, in order to get the ball up in the air and hit it farther I need a shaft that spins the ball more, not less. That's why that senior flex driver was so effective. I ended up buying a cheap "made for" shaft for $20, put it in my TL 310 head, and found I hit it around 270 according to the simulator with spin around 2800. Just for fun I hit a new R1, cranked up the loft past 11 degrees and tried it out. Effective loft was slightly higher, but spin was around 2200 with the "better quality" shaft and I hit it 5 yards less.
Those of you hitting the ball low... consider getting your swing on a launch monitor. See if you can find a setup that will work for your swing. It might not cost as much as you think!
Also, a great resource I found for info on this: http://www.tutelman.com/golf/ballflight/launchOptimize.php