Pretty much all of them that Ive looked into have relatively low BL. But Dan Wiggins, designer of XBL^2, says it doesn't have to, and I dont see how a second gap would necessarily lower BL otherwise. So I guess my answer is... maybe?
Keep in mind that BL is a static spec, when in reality BL changes as the coil moves (yes, even in BL optimized subs, albeit less). So a sub that has a BL of 24, but remains consistent through out its excursion range, could be said to have 'more BL' than a sub rated for a higher BL (say 30), but whose motor design means it loses it rapidly as the coil begins to leave the gap. Its like comparing two cars. One may have a HP rating of 300 hp, but you have to hit 7k rpm before it even comes close to that number, while another car/engine might be only rated for 270hp but displays that power along a wider range of RPM's. Which engine is more powerful? Really depends.