use 16 bits renders, there is plenty enough of info in them to be a good compromise between size/grade safe. Just remember some passes need to be 32bits.
as far that i know there is no rendering speed difference between exr multi layers with other method. But as i said when you use layering render from maya, some of the passes you can split from the main render are almost zero render time as part of the caculation of the main render.
Viewing the exr is indeed a pain. you can try to use RV or framcycler, but it is still slow. use nuke, it can play an exr file in cach. If you use zip compression, that will help. Rendering a quick jpg sequence con be good for check purpose.
for color differences in maya, check your setup are correct ! if you use a linear workflow, check you have the correct lut applied to your render view (in the property) there is a gamma to add on the viewer settings too. Make some tests, but what you see in your viewer must be what you see in nuke (with the same lut applied of course, let say srgb)
And don't forget that this linear process make that your render seen from outside will be different and bright, it need a srgb lut to be seen with the proper gamma curve !