It's great to hear that you've had a positive experience with your Ruckus equipment in the migration to 5GHz only!
I suspect that the main reason you've not seen a drop in RSSI is due to the extra TX power allowed with 5GHz.
2.4GHz is limited to 20dB EIRP in most regulatory regions. 5GHz has higher allowed transmit power in most regulatory regions. e.g. UK (and most of ETSI I believe) 23dB is allowed in non-DFS channels and up to 30dB in DFS channels. That said, most APs can't output much more than 23-26dB with built-in antennas. N.B. +3dB = double the signal power.
Also the free-space loss of 5GHz in practice is not noticeably greater than 2.4GHz, it's only when attenuating materials are in the signal path that additional loss (over 2.4GHz) is noticeable.
I hope that helps,