See my post here
https://mybroadband.co.za/vb/showthread.php/930982-RAIN-Mobile?p=21659967&viewfull=1#post21659967
I actually gotten some very nice info from Rain on how they run their network. In most cases they try for 4x4 mimo setup where the antennas allow. So a B618 should give you better speeds on the same band compared to a B315 if the tower is actually setup for 4x4 mimo and ignoring congestion obviously.
Once you have b3+b38 ca then the mimo gets bumped down to 2x2 mimo even if the tower supports 4x4 mimo.
Now for me personally CA works better even if you lose the 4x4 mimo advantage of a single carrier because of the sheer number of variables involved like how many users per tower, how many devices supports only b3 and what kind of bandwidth those people use and on what band. In my option I think CA just gives you an extra chance of getting better speeds because if b3 gets hammered 4x4 mimo is not going to help you.