Using the same host shortened L2M for single cr123 use and same 3.7 RCR123 battery, which would be brighter the correctly driven XP-G or the P7 which is supposed to be driven at 6 volts ?
It really depends on the cell and driver. A properly driven MC-E is a 9-10W device. That is asking an awful lot from an RCR123.
2.5A (Actually more as the voltage will sag like crazy on that load) is not a very good idea - cell life will be short. Those sorts of loads will mean replacing the cells often. Most protected cells won't allow such currents for long - if at all.
You'd need something like IMR cells to get anything like full output. You might get 5-10 minutes runtime out of it at full power.
If your RCR cells have low internal resistance, the MC-E might be brighter. But I'd expect the R5 to be brighter and run for longer.