Well, I hope you can justify it soon. You’ll wonder why you waited! The mk5 looks a great buy, and a great sub. You’re right too. 25 to 16 is indeed a more significant jump, but even 20hz at 100db or more is far more tactile and gut wrenching than 25hz at the same level. What I’m sayin is, don’t underestimate 20hz performance. IMO going from 25 to 20, is more impactful than going from 20 to 16, but also keep in mind a sub that can do 16hz, should remain linear around and well above 20hz. Much more so than one with an f3 of 20hz. Obviously we’re talking about movie playback.
I think cinemas don’t employ subs capable of really deep playback for a few reasons. Cost being the first issue, since most cinemas want to invest the least amount possible, and the second is availability of systems that can do the job, which is tied to the first issue. This also leads to more soundproofing($$$) as to not disturb neighboring theaters.
IMO, the content is there for nerds like us, who care about playing it back, and have rooms that allow for it with reasonable cost. I think too, that sound engineering is an art form, and lots of them are probably home theater owners themselves and “get it”. Conversly, most movie goers are clueless as to how low the system at the local cineplex goes, and don’t care. The ones who care, will either be disappointed, or wait until they can watch at home anyway. I know many who feel the only advantage to a commercial cinema is screen size. I don’t know exactly the spec for THX commercial subs, but iirc, 115db at 20hz is one metric.