@dstring 

Hey  Hopefully be able to answer this for you

I see the specs say you can have up to 8 SSID's for each band for a total of 16 SSID's.

Can I assume band steering will only work with up to 8 SSID's in total when each band has an SSID with the same name?

Pretty much yes, when you create a SSID it actually can create 2x SSIDs with the same name if you select 5g and 2g, one for each frequency.   Therefore a total of 8 WiFi SSIDs, but in reality its 16 as its 8 on each.

Band Steering isn't really anything to do with SSIDs par say, it simply allows devices to move seamlessly between the 2.4ghz and 5ghz when and if required, provided the same SSID is being broadcast on the other frequency. 

Also does having multiple SSID's affect the wireless performance?

Technically it could as there is more overhead to process the extra SSIDs, so should you totally flood the AP with 100+ clients all watching 4k videos..then in theory you might experience this.. however in short.. NO its not gonna happen :)

I have 4x SSIDs in my house for example, no issues here