Interesting. Always under the believe to never use AUTO anything when it come to this stuff
I guess it really depends on the environment you're running the AP's in. If you're not in a heavy wifi area, probably won't matter. If you wanna have a bit of fun with trial and error, run some iperf tests between the wireless and a gigabit wired client. That can tell you how much data youre really able to push over the connection.
You just need to ensure the gigabit wired side is of good quality and is actually providing more speed on the wire than the wireless can. So some testing between wired clients is a good place to start.
For instance, I had an Intel NIC and a realtek NIC to iperf test between. The realtek NIC was having a bit of trouble reaching over 500mbps, but upgrading it's driver got it to test over 500, then also changing out it's cable to a higher quality foil shileded cable got it to almost 700mbps. The Intel NIC did just fine. All the testing performed thru a trendnet green 8port gigabit switch.
Once I was satisfied that either client gave me consistent results, I started testing wifi speeds using a couple laptops. The exact speeds I was getting escape me right now, but they weren't bad on either band. I live in a more rural area, so not much interference around. I have everything set to auto.
Just keep a detailed log of your starting point, like OS level, driver levels, hardware specs, and what your initial speeda are...etc....as detailed as you care to make it.