You mentioned it's easy to dump RP before shifting, but have you considered the 1 to 3 (4 if full RP and Suddem Doom) GCDs that you might have to spend in order to waste as little RP as possible? Also, more importantly, it is invaluable to have some RPs waiting for you when you shift to Blood Presence to immediately use Conversion with.
Having to blow GCDs before shifting and having no RP after shifting will most probably spell death.
Glyph of Dark Simulacrum is another solid option that you're forgoing, especially useful against CC heavy teams.
You may also be interested to put this Conversion macro trick to your guide: http://www.youtube.com/watch?v=NWS9v73k0EU