The nice thing about a bs2 autocannon is you can move and snap shot without losing much accuracy. It also gives the unit a better way to take advantage of the av12 front arc. Which is easy to circumvent unless the chimera is sitting far back from its assailants.
Well, I was pondering a Heavy Bolter as It adds to the shots the Chimera has (I have been building lists with the Heavy bolter on the turret as well)... but with BS2 I also think a missile launcher for the frags may be the best way to add something more to the unit. Its not going to hit accurately (or hard) but it will add more templates.. which my current proto-list concentrates on bringing allot of..