I wouldn't be so quick to rule out the mobula as viable. True, they are very similar in roles with the exception that the Spire has a heavy weapon option. However what the mobula lacks in firepower it makes up for in versatility in guns. I normally have a merc, two artemis, and two caronades, all in a mirror config. This allows me to have 3 guns firing at long range, and 2 guns just in case a brawler ship closes the distance and gets too close. A spire with 4 long range guns might not have that close range defense. However, with the nerf to the brawler builds, I guess you could be right since no one's going to be getting within 450 meters of a good sniping platform any time soon!
Either way, hearing that changes to a mobula are inbound makes me very happy