To me any extra speed(in this case only a handful of kph) of going xl is not worth getting xl checked by the first mech that knows how to aim. Plus having a mech rush you on a red or open torso thinking it will be an easy kill get a bit of a surprise.
It's also a headshot machine. Dunno what it is about heavy gauss but as long as you have the aim point centered on the cockpit you often get a headshot. I kept thinking its gotta be hax when it first dropped and I was headshot several times by them. But nope. I'm not the best mwo player but damn if something isn't up with it for me to get semi regular headshots now. Might be heavy gauss slugs have a bigger collision hitbox.