It guess its not so much a question of restricting the higher frequency excursion, but adding distortion to it. A case in point can be seen in some of Ilkka's Round 5 results if you compare the % THD plots between the sealed LMS-5400 and the 2xPR LMS-5400. For every SPL level 105db and higher, there is significantly more THD between 35 Hz and 100 Hz for the passive radiator version (even though it has twice the volume).The loading at Fb ought not restrict excursion at higher freq, and would have certainly been noticed in ported systems.
http://personal.inet.fi/private/zipman/shootout5/diy tc sounds lms5400 18 pr 200l thd.png
http://personal.inet.fi/private/zipman/shootout5/diy tc sounds lms5400 18 sealed 100l thd.png