Currently, you only get the -2 bonus if the missile is aimed directly at the read of the target. If not, you'll suffer a +1 penalty PLUS the +2 side arc penalty.
My suggestion is a -2 bonus for the Rear arc, -1 bonus for the Side arc, and +0 for the Nose. Even if fired at the front of an aircraft, an infrared guided missile will alter its flight path to match the heat source, since the light emitted isn't restricted to a single arc. This will have the effect of simulating the difficulty of changing flight paths and pursuing the target, but still give the bonus of the missile.
EXAMPLE
Fighter A is at +0 heat. A tank fires an LRM at the Side arc of the fighter.
Current: [4 (Gunnery) + 2 (Medium range) + 2 (Velocity 2) + 2 (Side Arc) + 1 (unit at 0 heat) = 11]
Proposed: [4 (Gunnery) + 2 (Medium range) + 2 (Velocity 2) + 2 (Side Arc) + 1 (unit at 0 heat) - 1 (heat-seeking at Side Arc) = 10
