Although the xp per kill will be wrong, in theory you could test against other zones. I.E. if a lvl 25 mob gives, say 10% in Permafrost and a lvl 25 mob gives 15% in upper guk.. You could assume (perhaps after 10 tests) that Upper guk is superior to Permafrost.. Then move on to the next zone. Find all the lvl 25~ zones, all the 35~ zones and the 45-50 zones and test the percentage per even con kill. Again, you might not be getting that 15% per kill on Coirnav.. but you'll know you're maximizing your gains. Unless of course the optimal XP zone has sparce, slow spawns... Then you might as well take the 5% hit for the fast repops and mob congestion.
I was entertaining the idea of putting together a spreadsheet with my findings when mq2 works on test again.