Hello,
Although not an Epic player myself (being poor and living in a distant country...), I think that Epic is an excellent companion game to Battlefleet Gothic. If you look in the rules for BFG, it suggests planetary assaults use either Epic or Warhammer 40K rules. Unfortunately, every assault point you score is worth 2,000 points in WH40K, but only 400 pts in Epic. This makes Epic a much more viable choice to represent such cinematic moments (not that 40K doesn't have its place, for example, a small force is infiltrating the enemy command post and must destroy the enemy leader or a raiding party is tasked with taking out a critical supply chain that will determine the victor of the war...). I think that this is perhaps a good way to consolidate interest in your area, since Epic can be used to provide characterful scenarios (in the way the original rules seemed to guide the reader) and can be more easily balanced for such uneven battles (that is, giving players a chance...). People always find it cool that they can link battles in one game system to other game systems.
Specifically for the scenario, a good way to evaluate the effect of these turrets might be to imagine what might be happening in the space battle above—this provides an additional story-like element (which new people will probably like, especially when combined with a solid and sensible ruleset such as Epic) and helps players get into the feel of the mission. I would suggest that in some invasions, the preponderance of defence lasers does reduce the effectiveness of space vessels, but I always thought that their primary function was to prevent the landing of troops (which they do by shooting them and their parent space vessels). So, in such a planetstrike, the attacker would have a vanguard attacking the defence lasers. In other invasions, though, it might make more sense to scale the effects of the lasers with points/number of space vessels?
Just some (hopefully) helpful food for thought. Our prayers with those in Australia and all over the world in difficult weather.