With the relatively small amount of cargo it carries, it the liner is meant to make up for it with firepower.
Gunboat infernos can not take down a cruiser sheild as fast as a Cruiser inferno can take down a Gunboat sheild (which i think is what that slot should be).
The protection a liner offers is in exchange for 40% reduced cargo capacity. It makes sense for it to have GB turrets, to protect itself from gunboat that try to pirate it.
I am in favour of All GB turrets as the Liner carries alot less than other transports. I think we all know its a liner, but liners carry VIP's in a secure environment (supposed too) so i see no problem if you price the liner the same as the CT or the L.Train.
The liner is overpowered right now with the Cruiser sheild (IMHO) but if thats made a GB sheild, i would see no problem with liner. It takes double the time to earn the same amount of money with the Lux liner and i think that is enough of a sacrifice to garuntee safety on all of your trips.