Weight is pretty relative when you're dealing with a vehicle which comes with 18 armor and has 20 expansion slots.
I really wouldn't get too close to logic or reason when dealing with a big block of steel that goes 98% of the speed of sound.
Heh, they made a brick fly supersonic (the F-4 Phantom), so I'm pretty sure the Banshee isn't terribly unrealistic for 2070.
The Banshee is roughly equivalent to a light tank, isn't it (apart from the flying thing)? I can see it mounting the vehicle laser. The ship laser is what I find a bit problematic, and only a little bit. If it comes with an integral capacitor to store power for firing (enough for 40 shots), then I might see it being mounted. But, honestly, there are more cost effective ways to produce firepower. None as cool, I'll grant you.
@Casazil
Yeah, I haven't seen any alternative power sources available, yet. So it's electric or fossil fuel propelled. A rigger book (pleasepleaseplease call it the "Rigger's Black Book) would likely have more power options.