If this isn't merely a bandwagon response, I dont know what is. The OP didn't state any goals beyond wanting to try something different. //content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gif
What makes you think it will displace more air? You would be up-sizing to a 15, but downgrading excursion potential (by alot... 21mm vs 32mm). According to my rough math I did here quickly, displacement potential between the two would be pretty close.
04-05 XXX's are still awesome SQL drivers and you'll be hard pressed to find anything out there that is much better at getting loud while maintaining such impecable sq. The Fi BL is definitely not an upgrade, imo its a downgrade (needing to be amde up for with a larger cone).
If you just want something diffferent without a loss of performance (or needing more drivers), you could try the new xxx, JL W7's, 5200/5400 (always get thsoe two confused), etc. But honestly Id just look into reconing your xxx into a 15 or 18 before Id buy an Fi BL. The xxx reconed to a 15 would have alot more potential than would that new BL.