If you have a controller, you will be able to enable 802.11r/k fast transitions. In my experience, I've found a lot of mobile clients will roam more aggressively on a network that supports this roaming standard.
Even without such, you can try to use the "smart-roam" functionality. Basically it causes the AP to pretend to go out of range (e.g. ignore you and send you disassociation packets) hoping it will convince the client to roam. You can check the support documents on smart-roam (short story: Set a roam factor of 3 to 5 if you have decent network coverage, be on the lookout for unexpectedly dropped connections).
You should also try increasing the bss-minrate to 5.5 or so in order to encourage clients with a weak connection to roam instead of trying a lower data rate.
Also, since you are combining a 2x2:2 N (300mbit) with a 3x3:3 AC (1300mbit) AP, you probably also want the AC AP to broadcast a specific AC SSID... Almost no client roaming algorithm I've seen is smart enough to understand that, for example 50% signal strength on a 3x3:3 AC AP is worth more real world throughput than even 100% signal strength on a 2x2:2 N AP. That way, you can pin desktops/laptops known to be in reach of the AC AP to stay on it. I've had a ton of problems when I mixed N and AP AP's that my desktops would roam over to N AP's because they appeared slightly stronger.
At the end of the day, though, other than devices that support 802.11r/k, I have not found adding a controller to make a huge difference in roaming behavior.