Since 5% minoxidil regrows 50% more hair than 2% minoxidil, I suspect 15% minoxidil regrows 50% more than 5% minoxidil. I'm unaware of any studies. Xandrox is $80 per month. 5% minoxidiil is $7 per month. You can also apply it 3x per day if you want and maybe get the same effects as the Xandrox.
The other selling point of Xandrox is the azelaic acid, but Bryan and several others doubt it helps.