OK, just trudged through 'Sea Of Sorrows', at long last... I got the impression that 'plasma warheads' are probably just traditional missiles with plasma encased in place of a traditional exposive charge, because they're described as being dropped over a target.
Makes a lot more sense now.
Curious about why just plasma, on its own, would be regarded as particularly devastating, mind you. It's simply hot gas. Guess the authors just figured it sounded sexy!