It handles 5000 watts RMS according to http://www.audiopulse.com/products/subwoofer-drivers/lms-ultra , I'm not sure but I think it would be okay in reality.I don't remember the specs of the LMS-18 offhand but I highly doubt it will handle 6500 watts.
Edit: It has a recommended power handling of 5000 watts according to the specs on the website.
It's Basta!, found at http://www.tolvan.com/basta/ .I'm not familiar with that program you are using. What size cabinet did you model?
Excursion is not a problem in my simulation, there's a 1st order HP filter at 16Hz which will take care of that (just look at the curve).Ummm in a word no. The LMS (I have one) will handle more power thermally than nearly any other driver I can think of outside of straight up car audio SPL woofers. It will play fine wit a 5kw amp. However it will NOT handle that much power in the low bass. The driver will destroy itself mechanically with over excursion. Worry about mechanical handling not thermal.