[Kos-Mos]

I can easily see use for a 3-9x40 scope and 45° RDS.

In fact, that is exactly what I am going for with my XM110 (once it's done).

It's too long to acquire and focus a target under 100ft. with a regular scope.
And at that range, the 15-20° tilt you have on your rifle won't make a large enough difference. (You don't need to tilt the gun 45° to sight a RDS. When it's adjusted, as long as the dot is on target, that's where it's supposed to shoot.)

I personally will go with a 90° mount instead, with a docter sight on it. Easier to adjust the dot when the movement is in line with the axes. It also makes the sight sit tighter against the rifle, and maybe even tuck it "under" the scope profile.

For those that say a scope is useless in airsoft, I hope it's because you almost never play outside. Because it's incredibly useful, mostly for recon however.