IMP and IIW warheads are actually the perfect complement to the Cephalus's speed and spotting role. They degrade the target's accuracy, firepower, and mobility, making it easier and safer for the Cephalus to maintain contact until its Nova CEWS-mates take down the target. I wish there was a canon configuration with a couple iATM-3s or an iATM-6.
If there are fluff concerns about giving a Cephalus too much ammo and it staying out in the field too long, then apply iOS rules to iATM-3s. (Simulates the Fusillade on an omnimech while remaining rules-legal.) An iATM-3(iOS) will weigh one ton, so a Cephalus could carry five and a light TAG unit. Since iATMs have Streak capability, no iOS shots will go to waste. And 15 IMP or IIW warheads will cripple an omnimech or two.