Heavy and improved heavy Gauss rifles require a Piloting skill roll modified by weight class if a 'Mech that mounts them expends MP and then fires them in the same turn (Total Warfare page 136 for the heavy Gauss and Tactical Operations p. 314 to explain that the improved model works the same way). However, the modifiers listed there only go up to the assault weight class (at -1 to the TN), and the superheavy 'Mech rules in Jihad: Final Reckoning are silent on the matter.
What modifier (or modifiers) should apply to superheavy 'Mechs in this case?