First, which KK 50g shaft are you looking at, the Black DC SWF 50 or the the Ltd. TiNi 50? There is a big difference between these two KK 50g shafts. Assuming you are talking about the DC SWF 50, so it is a mid-high launch, mid-high spin, high torque (7.4°), 50g shaft while the Tensei AV Blue SWF 55 is mid launch, mid-high spin high torque (5.4°), 57g shaft. Beyond the basic information Titleist lists, you should talk with a fitter about each and even go through a fitting to see which might work best for you. You might also want to look at the Custom Options booklet to see if there are any other shafts you might like to try during a fitting.

media.titleist.com/.../Titleist-Clubs-Custom-Options.pdf

The KK is a mid-high launch and spin. the Tensei Blue is mid launch and spin. The technology is advanced enough from the 915 you may want to at least try both or get an actual fitting to get the best loft head with the launch and spin with any of the no upcharge shafts (from stock to custom order) for who you are today. Even at $399 you should get dialed in for max benefit.