We have 2 Unleashed R600s in our office setup with the following details:
Mounted on either end of the office, approx. 75ft apart
Just above a drop ceiling so they're out of sight
Hardwired to our main switch using PoE (Cisco managed)
Office is on the 5th floor with 1 other tenant
Building is in a very congested wireless area (~45 - 50 other networks discoverable, 20 - 25 that have strong signals.
They're not using their own DHCP server, our main firewall handles that (SonicWall)
Setup to use an external DNS server (Google's at the moment)
15-25 people in the office on a daily basis, mostly all using MacBook Airs or MacBook Pros
Most of our users aren't having any problems with internet speed, but we have 3-5 people on a regular basis that have very big speed issues and have to walk out of their office to get a connection or just constantly slows down.
We haven't changed any of the default settings on the APs and hoping anyone had any suggestions on how to optimize them for the congested area we're in.
If you have ChannelFly turned on, I'd suggest trying with it off for a while, because I've found those specific clients will disconnect or drop down to 2.4GHz when rapid back-to-back channel swaps happen.
At least do so long enough to rule out ChannelFly as a culprit, because your environment probably benefits from ChannelFly tech too, so I understand the reluctancy to do so.
My guess would be literally don't use it unless you have too. If all your clients support 5Ghz, disable the 2.4 radios or at least only broadcast your corp network SSID's on 5Ghz. You could still broadcast any guest networks on 2.4 for devices that don't support 5Ghz but if you can, move everyone to 5Ghz.
You'll likely lose some range compared to 2.4 so it may not be practical, the problem may also be that those 3-5 people are on the edge of 5Ghz range and are switching between 5 and 2.4 and having good speed on 2.4 but bad speed on 5. Ideally you need to determine what happens when one of these users walks out of their office to improve their performance (do they roam from 5Ghz to 2.4, or vice versa?)