only if used in a sealed enclosure is xmax a factor. There are many other factors that contribut to how loud a sub will play such as BL(BL is determined by the flux density (B) in the magnetic gap and the length (L) of voice coil wire in the gap. A higher BL will generally mean a speaker will have a higher relative sensitivity (efficiency). This doesn't necessarily mean that all speakers with a higher BL will produce a higher SPL. Often speakers with very high BLs have a smaller Xmax) this plays a larger role in ported enclosure where small xmax is ok, SD which is surface area of a sub which determines the amount of air moved, depending on the box you choose makes all the difference and also how they are powered.
The L7 would prolly be louder, but the Brahma wouldnt be far behind.
The difference in SQ would be night and day though. Once you hear a properly installed brahma, you will be comparing L7's to a monkey banging on a trash can.