The difference is a “never” testable theory would not be science, in my opinion. It could be considered theology or some subset of knowledge or speculative philosophy, but it is no longer in the realm of science. It moves into mathmatics or logic or some such. When energies get so huge that solar systems are required to generate them, I’d say we’ve reached that limit.
I’m a big fan of sci-fi, but I know it’s fiction, not science.
Never is a long time. I see what you are saying, though. Some parts of M-theory may well border on the philosophical. I’m a big fan of Sci fi also - few of my favorite authors are Larry Niven and Orson Scott Card (who, interestingly enough, is very religious). And it’s science-based fiction - just because the word Fiction is there doesn’t mean that some real science and very clever theories aren’t presented. Did you ever read James Blish’s “A Case of Conscience”? I’m pretty sure you would enjoy that one - I did.
http://en.wikipedia.org/wiki/A_Case_of_Conscience
Don’t be so doubtful of what humanity is capable of - look at what our imaginations have come up with (sci-fi is but one example) I can envision a day when we can harness the entire energy output of a sun (Dyson Sphere), or perhaps pull energies directly from higher dimensionalities. Maybe then we can test M-theory