Just tried the mod, Nvidia user here.
External view I wasn't getting any fps drop at all, in the cockpit I was though, enough to effect gameplay but not as bad as some. FPS dropped to about 23-25 fps but only in the cockpit when looking out at the clouds. I noticed the cockpit glass effects the clouds on the horizon, as if looking at them through uneven glass. Maybe this is what's causing the fps drop (that's a guess only).
Other than that, it looks amazing and really hope you find out what is causing the problem (or ED does).