I own the Dart pre-amp and the built-in phono stage is exceptional. There is something very elegant, logical and, potentially, sonically superior to this design.
An outboard phono stage requires additional non hardwired interconnects which will generally degrade/alter signal transmission. If the existing phono card were re-packaged in it's own chassis I think the addition of interconnects to connect it to your pre-amp would probably offset any gains from a seperate power supply and more isolated chassis architecture.
A completely re-designed and upgraded stand-alone Dart phono stage would likely yield incremental improvement, but at an extraordinary additional cost.
Upgrade your TT, Arm, Cartidge first. If you have any $$ left a Dart phono stage would complete your cost no object dream analog rig.