I run a Mauler with 2 Heavy Gauss and 4 Medium Lasers. If what I remember from MW4 the Faf had 2 Heavy Gauss, 2 Medium Lasers, and 2 Large Lasers (Because that extra 10 tons compared to the Mauler actually does help).
If I could throw in my own opinion:
Yes I understand the problems with a mech THAT wide, but we do have the King Crab, Awesome, Dire Wolf, and Warhawk. All of these mechs have some larger hit boxes, but if a King Crab wants to paint a giant target on its top, stand completely out in the open on polar highlands and get LRM-ed to death, that's the pilots fault, not the mech's. Each mech has its own role, that despite the hundreds of builds each variant can have, will always seem to settle into.
The Fafnir itself is one of my favorite mechs of all time. And I understand its problematic nature however, here's more opinion: Its an assault mech built for heavy gauss. Make it have zero charge time on gauss rifles. Let the Fafnir be the one mech that instantly fires its gauss like any other ballistic weapon with a longer cooldown, at the further drawback of no saving grace. If they are destroyed, they detonate like any other gauss rifle does 100% of the time. The mech already has a tiny firing arc, so its not going to whip around and instantly core the light mech who got behind him. A full alpha strike with this insta-fire only caps at 78 damage (50 for the 2 HGs, 18 for the LLs, and 10 for the MLs)
Change the Fafnir Gauss Cooldown to 8 sec to balance for instant fire alongside its other weapons and it has a overall DPS of 12.5 (weapon stats used found here:
http://mwo.smurfy-net.de/equipment, calculated the HG DPS with longer cooldown)
But hey, that's just my two cents.