With sine wave testing, at 16 mm peak to peak, the 4015 sounded noticeably “bad”, but at 12 was “OK”. Until 11 mm the sound quality is very clean. I think the 9 mm x-max rating is really conservative. That said, the speaker is so clean that with music you probably wouldn’t notice much distortion at all until the voice coil starts banging on the back plate.
If you can afford the larger box size, the extra efficiency down low will more than offset the excursion problem.
I built a 3.4 cubic foot box tuned to 38 HZ for a drum sub. Actual measured excursion with 100 watt sine wave at 30 HZ is 12 mm, which drops to 4 mm at 35, 1.5 mm at 40, 5 mm at 45, 7 mm from 50 to 70 HZ, then drops again.
Using a Crest CA-9 to drive it which does 550 watts at 8 ohms, have never heard any distortion or bottoming in drum sub use. The only protection I use is pulling the 25 and 30 HZ bands down on the drum mix EQ, and that the clip light only flashes on a very hard kick hit.
I have also played a variety of really low frequency CD music material, and found nothing that could “flap” the cone below amp clipping. If you are going to use an amp with double the speakers power, you would need to be more concerned, of course.
By the way, break in the cones before any testing. I ran the cone open air with a 20 HZ tone, turned the level up till it was about 10 mm peak to peak. I came back a couple (maybe four...) hours later, and could hear the dreaded clacking noise- the excursion had increased beyond X lim,(15.5 mm published) but the speaker was unharmed!