The Despoiler has stronger firepower than the Emperor. But 3 lances arent worth 35pts more than 6WBs. Its worth 12 more by Smotherman. Its worth 25 more by RC. Looking at the fleet lists, its definately less than 30pts. For what it does, it should probably be worth 10pt less than it is.
My formula puts the Emperor at 362.5pts, and the Despoiler at 392, or (384.5 with torps), a diference of 29.5pts.
Smotherman puts the Emperor on 365.5 and the Despoiler at 399 (396.5 with torps).
The reason smotherman comes out so high is because it massively over-values assault boats, charging the despoiler 28pts for them. I on the other hand, would charge more for 60cm lances firing in 3 arcs, which recoups some of the value.
As such, I think 390pts is a good price for the sum of the parts. There is a reasonable argument that as a support ship, the Despoiler should get a slight price break for its substantial off-side firepower, though not for its 'attack' variant.
Thinking about it, I'd make the Despoiler 380pts, with the torp option +5pts. This isn't because the torps are worth more (they're worth slightly less), but because the off-side firepower is worth more on this variant so it shouldn't get the 10pt price break afforded to the regular ship.
@Phthisis: I use an abeam-closing profile with my IN ships and it works wonders. I get to fire both prow and broadside at targets. The closing speed is about 75% of what it would normally be, but it makes up for that by allowing my battleships, battlecruisers and Tyrants to return fire on the way in. It also makes turning into the enemy fleet easier when I arrive. An Acheron on lock-on makes a mockery of 6+ prows, and a pair of them will easily cripple any 6+ ship that doesn't brace, even at long range.