@Asavoy
Sorry I probably misunderstood. I thought you were saying that Meraki as a vendor couldn't handle more than 3 SSID's. They can, but what your referencing isn't them saying you can't. It's them saying you shouldn't, which are two different things.
You COULD use all 15 SSIDs, but best bet is your wireless will start to suck.
SSID overhead can become a problem when you start to go over like 5 SSID's, which is why we recommend 4 or even better 3, or ever more betterer less lol. Each SSID will send a beacon, that beacon consumes airtime. The more you have, the more management overhead you have.
That applies to the 802.11 protocol, not any specific vendor. Same rules apply even if you have Cisco/Aruba etc.
To visually see what I'm referencing, see some of the links below.
http://www.revolutionwifi.net/revolutionwifi/p/ssid-overhead-calculator.html
http://wifinigel.blogspot.com/2013/08/its-well-known-rule-of-thumb-when.html
As for the firmware versions for each product train, I feel you trust me. However to be fair, any vendor will have bugs in their code. Cisco specifically over the last year or two has been pretty bad at doing any QA before releasing code versions for their WLCs. No vendor is perfect =(
Also, the 2.4GHz band your talking about only getting 3-10Mbps. That is going to be 100% on the environment, possibly configuration but doubtful since you sound like you know what your doing.
I can max out my Meraki 2.4GHz radio no issues. But I have had an office in a big city that gets less than 2Mbps on 2.4GHz. Had this with Cisco, had the same with Meraki. Not Meraki's fault that the wireless in that specific environment is terrible with interference, other neighbors. Its a giant mess, which is why I do my best to get away from 2.4GHz when I can. Just too crowded sometimes. I force any production SSID to 5GHz only when I can.
For your 2.4 issue I would do some spectrum analysis, how many other networks do you have in the environment. What kind of interference is causing the problem. Is it CCI/ACI, wireless, non-wireless etc.