Yeah I figured cost effective measures and timeline were the constraining factors lol aren't they always   You're right, in that you have plenty of time before the external temps (I think the worry starts around july).  There will be some low hanging fruit you can do as the heat encroaches.  Those metal doors would be priority to seal and insulate, that would be my first target in the coming months.  Running Insulation to the thermocline (heat/cool barrier, should be somewhere around the dark line on the concrete in your latest photo slightly above where the cool air outputs are located.  Both easy and cost effective solutions. 

Also an added bonus, those overhead, what I assume are wire runs, frames would work well for dropping vents above cold aisles if thermal turbulence prevents airflow.  Shouldn't be an overly expensive solution, if you find it necessary later.

I would suggest buying a bunch of cheap digital thermometers, if you don't already have a more sophisticated solution.  1 per rack section and a clipboard patrols/recording (I love low tech solutions) should let you moderately trouble shoot your arrangement.

Great timing on hitting this in the winter It will give you plenty of time to observe heat distribution while you still have plenty of cooling bandwidth before summer.  I hope some of this is useful for you and wish you the best of luck.  If you ever want to bounce ideas, just send me a PM.  I love this stuff.  Keep up the good work.