Hi, Mike.
Great job with the spraybooth. Looks like a lot of thought and skill went into the construction.
Couple of thoughts:
The front plexiglas that you cut out, is that used to seal the area where the 2 filters and fan is located? It's important that area be fully sealed. Any leaks can cause significant velocity loss where you need it most. You might want to consider doing a leak check of that area of your system. With the booth running, simply light a candle, blow it out, and watch what the smoke does as you pass the candle near the filter/fan area. Any air movement in or out indicates the need for some weatherstripping and/or caulking.
You also mentioned installing a "low micron filter" to clean the air. Unfortunately that filter, nor the ones you have installed now, allow for the capture of the organic vapors generated from the solvent fraction of your paint. I would recommend purchasing an activated charcoal filter and installing it where the filtrete filter is now, with a paint filter/blanket on top of the charcoal filter.
The installation of an activated charcoal filter is suggested only if you don't plan on exhausting your air to the outside. The first picture would indicate no exhaust ducting in place. If you've already rigged your exhaust ducting, and everything is going outdoors, disregard my suggestion.
Hope this helps,
Gip Winecoff