Hell NO should it get S3 Lances! Not that I think that would make it overpowered (15WBe), but it is not meant to be a super-dauntless with L/F/R Lances and AC, it is a mini-dictator.
I think lastspartacus's suggestion was eminently reasonable. It NEEDS the torps in order to give it a chance of getting a reload ordnance special order, and given 4wbs (the firepower of a sword) in addition to torps, it would have 16WB equivs (and that's counting the LBs as 6WBe despite being more like 4WBe offensively), compared to a dauntless's 17WBe, and the Endev/Endur 20Wbe - hardly overpowered.
It would still be undergunned, but it might just carve itself enough of a role to make itself useful with that load out.
As for why 45' turns is fine - These are mini line cruisers. Their role is identical to that of a line cruiser. The Imperium builds them because it is vast and it needs to show the flag of a capital ship in more places than it would otherwise be able to with standard line cruiser. It is still effective in the line of battle, and it can head up a flotilla of escorts in standard combat missions just as well as the dauntless can (better even), and in defensive situations is also worth more than a dauntless. Its role is that of a line cruiser, and it doesn't need to be given additional roles by making it more manouevrable.
As to why the player would take them instead of a line cruiser: They're a cheap way to cover your bases (another reason why the endeavour-endurance/defiant link needs to be severed), adding more WB, lances or LBs if that's what you need. They're also an alternative to the line cruisers, if that's your preference. You may prefer to field more ships than fewer - as long as they're viable alternatives to line cruisers then this will work. It's also fluffy, which again will work as long as they're viable alternatives.