You can test for enlarger light evenness with a spot meter, aiming it at the center, corners etc from the lens axis. Don't overcomplicate this.
Regarding the Mod54, I'm testing this product now as it happens. One thing I don't like already, the tabs that hold the film extend too far out into the image area, causing tiny scratches on the base side of the film as the film slides up and down in the holder during inversion. I'm also finding I need significantly more than 1L of developer for it to work properly (not a problem, but worth noting). Lastly, I would suggest doing only two sheets at a time in this thing rather than 6. Testing is preliminary so far, but I still find tray development in a thin wire slosher to be the easiest way to get uniform development, including Pyro. The only thing better than the slosher is doing a single sheet in a tray. Of course the Jobo is reputed to be excellent, but I've never used one.