Having used this to brutal efficiency I'm wondering if it's too good, or at least biasing marine behaviour.

It is a far better option for a marine to go on guard than to fight in melee in their own turn, especially if they're a combat character (sergeants and lightning claw guy).

I found myself always putting the lightning claw model on guard rather than moving forward and actually attacking because the advantage is so great.

So I was thinking, what advantage does shooting have that prevents overwatch being too good? Sustained Fire.

What if you give melee (for marines only of course) +1 for subsequent rounds of combat, sustained assault.

I don't know, but marine melee is really more rewarded on the defensive rather than offensive, which I find a little wierd with something like a lightning claw terminator.