Yeah, after running some calcs myself, it would take a minimum of a +3 boost to have a chance of OHKOing a Heatran, +4 to guarantee it (both of these scenarios done under the assumption Rocks are up, if they're not then it'll need an extra boost for myprior statements to apply), so... not the ideal Metagross is not an ideal check to Heatran without more force. I'm still not letting go the viability of PuP Metagross because I'm a stubborn turd, but running Scarfchomp, or Terrakion if you have enough other stuff to resist EQ, on the same team is probably your best bet.