Wow. The Craftworld picts are awesome, and make me feel this was probably not the best time to put together a post talking about light artillery. Oh well.
I've been thinking about various forms artillery squads could take, limiting ourselves to the techs we've currently got access to.
For vehicles, I'm sticking closely to the original slot layout, because we've since learned that the surcharge for radical chassis redesigns can get quite expensive.
Infantry squads could come in six or twelve, with VGW (6 EP), a Heavy Fusion Mortar (18 EP), and maybe whatever our conversion fields look like. Suncaster Pistols cost another two each. Maybe an auspex for the squad. This is a squad that is too expensive to be anything but elite - solidly over 25 per figure, even with our discount, Fusion Mortars are pricey. Infantry carrying them is still the cheapest way to get lots of heavy mortars on the battlefield, though.
Alternatively, we could stick them on Heavy Jetbikes, and have a squad that might not have as many mortars, but is quite capable of delivering them anywhere on the battlefield. (It's not worth the slot charges to get them onto a regular Jetbike, I think.)
Here we're looking at 1 heavy mortar at 18 EP, 2 sunblaster rifles for 6 EP each, a grav shield, and two crew with VGW and sunblaster pistols (6+2 each).
Lastly, we could bump up to something capable of carrying vehicle-scale fusion bombards.
Light grav vehicle, buying a single extra H gun for 6 system slots. 2 H guns could either be mortars or close defense guns, maybe our graviton sheer weapons. 2 V mortars at 32 each. Six system slots get a holofield and a grav shield, leaving one slot that probably stays empty for some future piece of equipment - possibly boosted sensors or a targeting computer, if we can unlock those.