[RC-Monster Mike]

172oz/in at 6 volts is a high torque 1/8 servo - in fact only a scant couple servos with this power were even available just 2 or 3 years ago. 100 oz/in is plenty for a 1/12(and will be much easier on the ESC BEC). That said, I would think the MMP BEC should handle the load of this servo on this small vehicle as long as the servo is working as it should(worn gears will bind and cause high current draw in use, though).
Likewise, and standard linear BEC on a 3s rated ESC will be best served with an external when 3s is used - just the nature of the linear BEC and not specific to the Havoc.
Lets face facts here - the MMP is a 1/10 ESC intended for 1/10 vehicles - many folks here will be using it in their 1/8 buggies - the MMM is the Castle ESC intended for 1/8. The MMP is a cool little ESC, but it is half the MMM in reality and expectations need to be realistic. :)