MatthewASim
Are you set on staying with the 915fd? A slightly more recent model can give you the ability to throw weight in the toe and mitigate the lefts. I've been a big fan of the M5 5 wood for that very reason. The last two Titleist models offer similar and you wouldn't have to change adapters.
I would imagine the X100 feels better because the butt section should be softer than the graphite shafts you listed. If you want to go 100g, you could try the Project X 10A2 if you can find one. Those were very popular for the heavy weight crowd.