you could try asking in the 1/8 scale electric thread for more response.
I know of people who have used the 1512 in 1/8 scale truggy on 4S and were quite happy with the motor in it. The 1515 will provide more torque, and probably run cooler at that, so more people will recommend it as a "truggy" motor based on this.
If you need guarenteed 20 minutes, your 5S 5000mah setup should give you a great shot at it, more so than the 4S 6000mah pack I use.
< Durango DEX8, DESC410V2, DEX410V5>
< Team Associated RC10B6D>