Without modelling, I would say either 15" would run out of Xmax before they get anywhere near their thermal ratings - the 12" 12LX60 I'm playing with does (although it is smaller, Xmax is larger and power is lower), so I expect a waste of power capabilty (which would help with power compression anyway I suppose). Of course a lot is dependant on how low you want to go.

The cone strength of the Beyma was also a concern to me with the 12", but the larger voice coil diameters should help stiffen the cone somewhat. This will be an area to watch when my prototypes go into service (hopefully within month).

The talk of maximum output makes me wonder what the goal is. If the size of the box is small enough to be handled, then 6 or 8 may be required in a stack to get flat response and maximum efficiency - if each box is going to get 1500W, then is the goal to have the audience throw up at the first kick drum beat?? Somewhere around the 130 to 135 dB at 1 metre maximum should be plenty..

Cheers
Graeme