I just did a quick estimation and I think there would still be too much current direct drive even with a quad XHP35. From the forward voltage curve on the Cree data page, I estimate the voltage is about 13.2V at 2.1A. With 4 low IR cells in series, the resulting current will be significantly more than 2.5A per LED. If you used high IR cells and/or added resistance somewhere else it might work.