Would using c++ instead of blueprints grant any significant performance gain on mobile platforms?
No, it will not. Rendering shaders is what slows old mobile devices on android.
So unless you are making heavy calculations in blueprint, moving to C++ will not add much.
not only shaders as having couple of billboards with unlit material still can get choppy 
Fillrate, texture bandwidth and pixel shader processing power is the biggest bottleneck on low-end Android. The terrible OpenGL ES drivers don’t help either. If you’re billboards with alpha blending, you’ll stress fillrate if you have a bunch of them overlapping each other. You’ll actually get better performance by making opaque polygonal cutouts instead of a single quad with an alpha blended texture, specially if it covers large areas of the screen and overlaps a couple times.
ok, does paper2d use quad or makes a cutout?
You can make cutouts in sprite editor. Manually and/or Automatically