Cover is usually a Setback die or two. Nothing too hard to shoot through for a trained gunner. I don't see needing Computers checks for targeting combatants that are actively shooting, but if they want to go all hide then sure. In general, most things that make shooting the speeder mounted weapon harder are going to make shooting any weapon harder. In some cases, the planetary weapon might even ignore lighter cover since it can blast right through it.
Let's test it....
So you're in say... one of those weird flying Thighmaster gunships from Clone Wars, and you're attacking some Mercs in Mos Espa at... dusk... We'll assume you know where they are (an NPC buddy of yours commed it to you right before they shot him, so they also know you're coming and are taking cover).
You are at extreme range and want to fire your autoblasters at these dismounted fools. So I'd say your dice pool looks like...
2 Purple for base difficulty
Increased once because there's lots of small one story buildings in the way that might block your LOS outright
Upgraded once because you're in essence attacking a civilian town, so there's a lot that can go wrong there
Added 2 setback because the Mercs are actively using the town's features as hard cover.
Added one setback because the lighting isn't so great
Total: 2P, 1R, 3Blk
If you've got a Skill/ Ability ratio of say 3:2, against that you'll only be hitting about 1/4 of the time, maybe less. But I do feel the GM could easily justify reducing that difficulty as you get closer to you targets and can better pick them out.
It goes back to what I was saying, if those mercs were out on the flat at mid day, the difficulty would probably be just plain 2P, and they'd deserve whatever fire you want to rain upon them for being so dumb as to get caught in the open. So it's up to the GM to just give it a little thought, and design encounters that aren't that cut and dry, because doing something as simple as moving the mercs into town, and dropping the suns down over the horizon completely changes the dynamic.
Plus it encourages more interesting encounters anyway... In the out in the open situation you blast them in a single strafing run before you even get into range of their weapons. In the Mos Espa scenario you can either sit back at range and level half the town as you bombard these guys from afar, or you can cowboy up, move in closer, risk getting shot at a bit, and run a really dramatic chase as your Thighmaster pursues them through the streets and across the rooftops...