These substantial looking plates are 5mm because it's the same thickness as the clamp plate and it's thicker than 3mm. If you remember all the way back to the start of the year, I had to use washers because of the DDEs (aka BMGs). Since I was planning on inverting the DDE's closest to the hotend, I thought I might as well correct my earlier error. 

The uni only happened to have M5 x 20mm bolts, so I used the M5x5mm brass insert strategy to "shorten" the threads to M5x15mm whilst undergoing the installation:

It was much easier to install the DDEs and they feel more solid/secure. I've been able to shorten the bowdens E0, E1, E6, E7 by 5cm, down to 30, 30, 30, 35cm respectively (about 14%). Additionally, it's much more ergonomic to reach the large gears on those extruders now.