Yes indeed. Probably the #1 biggest use for STUN short term is going to be SIP. It seems like not too much information has to go thru the known reachable machine. Maybe just about the same loading as a DNS server?
Wrong. For a host to receive external calls, the binding on a NAT box must be active all the time that periodic refresh is required.
OTOH, DNS query is necessary only on demands when a host want to initiate external calls.
Masataka Ohta