With the belt alternators like I have, the 520s are pretty similar to 470s. I'm not a fan of the front alternators because of some nasty failure modes. But they do still burn a bit more fuel for a bit more speed, so it works out.
RAM's views on LOP only apply to you if the engines are under their warranty. However there's no reason why they need to overhaul the engines if you buy a used plane with their conversion. For ROP, one friend with a T310Q RAM I was getting 175 KTAS on 30 GPH.
As far as cost vs. a Seneca, the Seneca will burn a bit less fuel, but go a good bit slower for it. The 310s are quite good hot rods, and the Senecas aren't. A RAM IV T310R is absolutely as great as it's made out to be. Our 310's previous owner has one now. It's the only 310 I've seen/flown that makes ours feel slow.
Eggman, I've got some other thoughts on your numbers, but we're at the airport now.