I did the deployment last weekend with 20 fat AP 650 (single radio!) In an event for 1000 people, where 700 of them used the Wi-Fi.
With low density, my retry index was spinning around 7% (acceptable). During peak hours of people, I saw through the NSight an increase of 40-50%. From what I've seen, even the device with Wi-Fi not connected, it causes pollution of beacons (probes circulating in the network).
I did the whole project on Ekahau, using the concept of small cells, minimum bit rate of 18 Mbps, 802.11 k / r and the best resource of all: load-balance group of clients between APs. It worked! haha !. That's where WiNG's big differential comes in!
Unfortunately I was not in place during the event, I was not able to do Layer 1 & 2 troubleshooting (Chanalyzer, OmniPeek, etc). I was curious to debug the frames, make adjustments in the beacons (DTIM interval), and follow the evolution in NSight ...
Anyway, from the comments I received from the users, I had no serious complaints.
As Tomasz stated you could establish airtime utilisation on 2.4Ghz and look if alternate power/channels might help, did you check for issues such as hidden node, near/far and ACI? Retries are part of the 802.11 protocol, there isn't anything you can do to tune this specific behaviour other than to try to ensure the packet delivery happens first time as much as possible, although you will never eliminate them.
The best solution however is to move as many critical devices onto 5Ghz as you can, typically guest isn't critical although it might be in your environment, some ideas:
Band steer 5Ghz capable devices away from 2.4Ghz, freeing up 2.4Ghz and giving 5Ghz capable devices a better experience.
Have the corporate SSID only on 5Ghz, create a new corporate ssid and call it "Corporate Slow" and put this on 2.4Ghz - corporate users who connect to this SSID can be identified and the reasons why investigated (old hardware, bad RSSI for 5Ghz.)
Keep guest on 2.4Ghz only
The above does require your network is designed for 5Ghz coverage throughout.
Unfortunately there's no quick and 100% solution for every such case.
2.4 GHz is polluted, that's right, but how much polluted is it in your environment?
Adjusting the powers, channels and clients per AP count according to actual RF domain spectrum is one option, as well as adjusting the minimum basic rate if possible.
Looking for APs and other 2.4 GHz devices that should not be here is another option. I'm not sure, but maybe some devices could also be a reason for this?