ive seen plenty of 5m's with BHG's, and usually with poor head bolt tension...
nowhere near as regular as 7ms (i suspect the 5m comp ratio and cam profile, increasing age and leakdown - as well as the fact people dont 'expect them' to go as well as a 7m 'should' - help deter detonation and crazy cyl temps and pressures)
checking the head bolt torque is an easy and effective thing to do on any engine tho