Use of virtual ground to split ground loops?
Do you just want more amplitude out of your signal?  Why go differential over coax?  

What is P4?  I'm assuming the shield of whatever coax that is (BNC, SMA) will be exposed to the operator and any equipment.  I try to drive any exposed metal to ground, so I would feel a little uneasy about your setup.  Can you add another cable to drive the other half of the differential signal?  I've seen it done like this in industry.  I think you'll continue to fight grounding issues if you stick with this setup.