The PG will be more efficient than the BXi606 by matter of topology. What coil config on the Rl-p? If its a dual 4 you might be able to mod the PG for more power at 2 ohms.
the xenon 600's i have benched (now almost 20 of them) never did less than 830 watts just fyi i own 2 that i run right now one does 896 the other does 879+ so i would go that way.. they are fast becoming scarce though get one soon