Just the opposite, I've gained just over 10 from my 913.

The 915 D2 are the longer club than the 915 D3- that's what I've experienced when I was testing, but then I went with the 915 D3 because of other reasons for me. Anyway, I changed to 915 D3 because distance was one of the big factor from what I had before including other drivers that I've tested at the time.

Did you take your G15 with you when you did your fitting for the 915D2? My fitter insisted that I bring in my old driver when he was fitting me. I lost about five yards but the accuracy was unbelievable and after playing with it for a while and getting use to the new shaft I am now driving it past my previous driver. I went from a 910D3 to the 915D2. With the new head and shaft (Rogue Black 70)my spin is down 2000-2500 and my carry is longer and higher. Give it a little while on the course.

I wasn't comfortable with the speeder shaft on my 915D3, set for C2 and I'm still not but it's getting better. I've hit a few nice pokes with it but just for snits and giggles, I flipped out the speeder 515 and put my Diamana Ka'li S in and took a few pokes with it, but meh, it's about the same.
Confidence is key my friend. If you don't think you're going to hit it long and straight, you're probably not going to...
Step up, think about the ball flight you want to hit and let it rip (and of course I mean as much a ripper as your swing will allow) and when your swing speed is normal and not tentative, I think you'll find that you will gain a few yards at least!

I'll agree with Chuck on this one all day. I always take my old clubs to a fitting, and it's very helpful, especially on a monitor....

Also agree with Chuck. Starting with a 6 year old driver, a properly fit 915 will improve distance and/or forgiveness.