Seems like that could be a bit on optimistic side...
...at least in simulated theory
Seems like you will be able take the 21DS115-4 up to about 61V in that 4.5 gross cu ft enclosure while staying within reasonable excursion...
True of course but it's only after response shaping of the cabinet & driver system's passband that it becomes truly useful, especially a strong pro driver in a sealed cabinet. This is the same as above after LTW F0=58Hz, Q0=0.35, Fp=20Hz, Qp=0.707:
Max SPL 112dB, F3=20Hz, F6=15Hz and F10=12Hz. The borderline excursion at around 10Hz should be mitigated by electronic roll-off elsewhere in the signal chain:
Again, only theoretical anechoic response in the simulated world of course. In-room response will of course require further ("different") EQ to flatten the response. Nevertheless, if you stick to only reducing gain (without any boosting) during the EQ process, the SPL capability at the low end of the passband (e.g. ~20Hz), in-room net after adding room gain and deducting compression, will ultimately dictate the performance throughout the whole passband (e.g. ~20-100Hz).
Also subscribed and looking forward to your test results, Ricci! A couple of these drivers are potential candidates for my own system - unless the bigger brother 6" VC is expected to follow soon...
EDIT: Simulation file (WBCD5 Excel XLS file) attached for anyone interested in playing around.
Woofer Box and Circuit Designer 5.zip