Land mattress needs to come early when you consider it gets shut down by the Stuka.
Really it's dumb to compare with the Calliope, with the land mattress all you need to do is barrage it with a stuka or snipe incediary shot and it's dead.
It fires a lot of shells and fuel wise is cheap, but it's short range and 2 min recharge means it's easy to catch out with a few LMG bursts. Also bare in mind it costs more MP than the katy or werfer.
All in all I would rather have a non doctrinal mobile rocket platform like the katy/stuka/werfer
I'am really thankful that you have changed your signature dude