Some thoughts about Bluesbucker in the neck...
I don't have extensive experience with P90's but I have played my share. Bluesbucker to me sounds like a sweet, delicious P90. Maybe not quite as gritty as some. In parallel it sounds a *LOT* like position 4 or 5 in a strat and is quite a bit brighter than in series. This pickup is very sensitive and responsive to volume and tone adjustments, even more so in parallel. And coil tapped is in the telecaster realm.
If you are considering a Duncan P-Rails, then you should also consider a Bluesbucker. I have a P-Rails in the neck of one guitar, set up for switching between series, parallel, P90, and single coil. If you set up the Bluesbucker to switch between series, parallel, and single coil, it definitely covers the same ground as the P-Rails, with the only exception being the full series humbucker mode of the P-Rails. The P-Rails is a hot, beefy humbucker with a considerable amount more balls than I would have expected. But if you can do without the humbucker balls, then Bluesbucker is the ticket.
I think the Bluesbucker just sounds sweeter than the P-Rails, though. I know that's a completely subjective thing, but I don't know how else to put it. If you're one of those guys who doesn't believe that volume and tone knobs are necessary, then either the Bluesbucker will teach you that they ARE, or you will probably not be crazy about it. Also, personally speaking, having a Bluesbucker in the neck completely solves my previous need for a strat, because it will do absolutely everything I'd want (and more) from a strat.