Re: Arena Map Lab
This has a much more solid and coherent feel than your last map.
I think as far as the layout goes, things might be a bit too spread out. It felt like I was spending all my time bounding down passageways towards my next kill, which is a shame as some of the rooms have interesting height variations and cover in them. Maybe generally shortening the corridors would help this.
A more interesting exercise in layout tinkering might be stacking up some bits of your map so you can have some more vertical connections between different areas, make navigation more varied. Aim for it to feels like you've got rooms on well-connected two floors rather than one floor that meanders up and down a bit - I'm not suggesting one tall pile! For example moving the lab area with the windows so it is underneath the section with the single long bench and then rebuilding the connecting bits, with a connection to the doodiehead graffitti room from the window lab, etc. (That doesn't entirely make sense without pictures I know, but don't worry, have a think about how these things could fit together, I bet you can come up with a more interesting and more densely packed layout than that quick suggestion anyway). Maybe some stairs as well as all the slopes?
There seems to be a problem with the game often wanting to fill the passage junction just outside the doodiehead graffitti room with lots of enemies - maybe it thinks it's a fairly nearby point wherever you are in the map (I got the buildup when I was bunnyhopping all around the outer circuit and wondering where the dudes had gone). But if there's a lot of spawners around there you might want to spread them out more.
Not sure if two megahealth secrets is a bit much, maybe convert one of them into an ammo cache? Probably the one with the blobs, it seems slightly easier to find.
Maybe for the lab entrance you could have some (bulletproof) glass doors and windows and show a little yard outside with a bit of vegetation as a break from the industrial stuff.
Keep at it, it looks promising.