The Subach HL-7 is more effective than the Shivan mega Laser IMO.
I still wouldn't try to go head-to-head against a Seraphim armed Megas or even Heavys.
What the GTVA needs to try and do is either replicate the power and rate of fire the Shivan beams have, or greatly improving the range of their beam weaponry so they can effectively act as artillery (thus perhaps making TAG missiles worth using in a nebula).
The problem is the Shivans always have at least 1 or 2 escort wings for their warships with backups if necessary. Shivans are also better at subspace jumping, so you'd have to disarm AND disable their vessels. Add to that the fact that they just throw something bigger at you when you kill them (kill a cruiser, they send in several more, kill those and its 1 or 2 corvettes + fighters, mess those up and they call in a destroyer and lots more fighters, take that out then a juggernaut comes after you...) so they whittle you down via attrition.
Point is, Shivans excel at offense no matter what in the end. Any serious assaults they do, the GTVA always gets forced to retreat. On defense they are weak, but the fact remains that counterattacks are still attacks.