With XBL^2, performance is extremely sensitive to the coil length with respect to the top plate geometry. The intent of XBL^2 is to achieve linear BL...if the coil is too long, you will have substantial ridging at the outer limits of the stroke.
Pretty sure it was limited by the basket actually...on the Mk1 and Mk2 Brahmas, both the 4 and 12 spoke baskets were limited to 32-34 mms reward clearance, at that point the spider would be sitting on the top plate.
IIRC both the xxx and Brahma, had a rating of 32mms of Xmax,