I purchased this same set-up from Reef, using the 45 degree finder. First off, I HIGHLY recommend it. The standard Nauty finder is OK and useable, but the external finder is like going to the moview by comparison.
It does cover the LCD screen a little, but, once you use it, you will not be using the screen. I have done two dives and shot 200 images and videos, and not once have I felt the need to look at the LCD screen. Remember that the EVF can show you all the settings so even if you have to go change a setting in a menu there is no need to use the screen. I believe that, because of the more compact dimensions of the EM5 housing (ie the viewfinder is close to the ldc screen) that even the straight finder will also block the screen somewhat.
You might want to consider the 45 degree unless you already have the straight finder. The EM5 housing is very compact, such that using the 45 degree finder is no effort at all as you are not moving a huge bulky housing underneath you. It's hard to describe but very evident in use. I have no issue using the 45 for wide angle shots. For macro, it is far superior to the streaight finder as you can have the camera below you and stay off the bottom.