Titleist drivers typically have a swing weight in the D2/D3 range, hence the "heavier" feel. Since the swing weight is adjustable with different weight shafts and bezel weights, I'm sure you can work with a certified Titleist fitter to come up with a head/shaft combo at a swing weight that gives you the light feel you want.

I was driving with the XR and just switched to 915D and cannot belive how much distance I'm getting now. Driver does not feel heavy in the least run don't walk to get a 915 in your bag. #TitleistTheBest

Many times demo clubs are not swingweighted correctly which could make them feel too heavy. Here is a potential solution for you....I would consider a Titleist 915 D2 head with a 50+ gram shaft (Aldila RIP Phenom 50 or Mitsubishi Diamana D+ White 50; Stiff Flex as you suggested but you could also do a Regular flex if the shafts felt too stiff) with D0-D1 swingweight. If the driver feels too light then you can first try to raise the swingweight to D1-D2 (just add some lead tape to sole of driver). If you still feel it is too light, then step up to any of the numerous 60+ gram shafts Titleist offers. This will certainly put you into a driver that fits you very well...if you are still not sure just get fitted at any Titleist fitter.

Hope that helps!

Great advice, I think I have a 50 g shaft right now.