They can take 4 splinter cannons. It's still expensive in points, and you really aren't getting that much output from them unless they are within range of with their sharcarbines. At that point, they probably moved, so you're only getting one extra poison shot with the cannons. Thumbs down to that option, especially considering the alternative is 9 Reavers with caltrops.
Shredders are a interesting option. On the one hand, they are short range (though better than the melta on heat lances). On the other hand, they are much like caltrops, scoring several S6 hits, but they can also be used against vehicles (lighter ones, or rear armor). The shredders are also cheap, but the Scourges are still a lot of points for what you get. Unless you want to maximize your cheap S6, and you already have Trueborn using shredders for far cheaper, then I wouldn't consider this option either.
Straight shardcarbines is probably their best option, since you're paying for them whether you switch out for another gun or not. Just try to kite things around and use those 3x splinter shots as much as possible. This is where they can actually be decent. Causing 10 wounds from that range means they will take out around 3 MEQs or 6-10 GEQs per volley, which can itself put them beyond assault/double-tap range. Are Reavers still better? Probably, but they are also more expensive.