I actually disagree with the fact that either of those composers were/are particularly important to the continuation and evolution of modern, classical music, beyond the aforementioned teaching of Messiaen.
I think that this is fair comment; the value of any individual composer's work is not necessarily reflected in the extent to which he/she is, or is thought to be, influential on future generations.
I've always thought of Messiaen in a similar way I think of Shostakovich and Stravinsky; truly great composers, but ones who hardly anyone really emulated or extracted inspiration from in their own writings.
I cannot agree with you entirely here; Shostakovich certainly exerted some influence for a time on youger Russian composers (although I agree that this was not especially widespread or long-lasting) and the influence of Stravinsky has been rather more prevalent (and lives on even today in composers such as Andriessen).
While certainly there are a couple splatterings of composers here and there like Takemitsu who did adopt a heavily "Messiaen-inspired" voice, I think that, among what are now the major composers, they are extremely few and far between.
Takemitsu is undoubtedly one of the more obvious examples, but I would submit that the influence of Messiean, such as it has been, is rather more subtle; ask almost any French spectralist composer and he/she'll probably admit (if being honest!) to drawing on Messiaen to some extent at some point, even though his/her own music doesn't obviously "sound" like Messiaen's.
The composers of the 20th century that I would say have been possibly the most influential are Schoenberg, Debussy, Ravel, Hindemith, Cowell, Ives and Gershwin for Pre-Darmstadt,
I rather have my doubts about Cowell here (and that's not a value-judgement of his own work, merely a doubt about the extent of his influence, since his music is rarely heard or discussed outside US and, even when he is discussed, it is often in the context of American mavericks such as Partch rather than as a composer central to any identifiable American musical tradition).
and then Cage, Carter, Xenakis, Stockhausen, Ligeti, Bussotti, Ferneyhough, Boulez, Vivier and Reich for Post-Darmstadt and 21st Century.
Again, I think that you risk confusing importance with extent of influence. Carter, who has found his own ways with immense difficulty over a remarkably long gestative and maturing period, has never really fitted into a particular persuasion and it would be hard to detect specific widespread influences from his work. I also have my doubts about Bussotti and especially Vivier in this regard (the latter being somewhat less well-known than the remainder of composers on your list here). Ferneyhough's influence probably has at least as much to do with his teaching activities (which he regards as being of central importance to his work as a composer - perhaps more so than most).
I think Messiaen's harmonic world he created is unto Messiaen himself,
That's absolutely true; as he matured, some confluence with Alain, Langlais, etc. is undoubtedly detectable (as well as a whiff of Dukas and Roussel), but he did really become something of a law unto himself.
and I believe some of the theory he is often associated with was being explored well beyond what he was doing, before he was doing it, and that the composers who were really utilizing it were extracting it not from Messiaen, but from his students who seemed to have almost instantly taken it lightyears from what Messiaen himself was doing with it.
I think that you'd need to flesh this one out with some explanatory detail! - and one could likewise argue that Skryabin, Roslavets, Hauer and others wwere experimenting with quasi-serial ideas before - and independently of - Schönberg. I'm also less than certain that your statements here are wholly compatible with your previous sentence about Messiaen's harmonic world.
Like Ferneyhough, Messiaen had many students. Of the other composers you mention in this post, Schönberg probably had a greater number of students than most. The extent to which such widespread tutelage may be reflected in the influence of the music itself is inevitably debatable and variable from one composer to another. It is also important to consider for how long any such influence may be seen to prevail - and where it pertains geographically.
So, it's a big and interesting subject which is far from easy to see in black and white in most cases, but I think that we can agree that the extent to which any composer's music may be seen as influential is not necessarily an indicator of its value or of its expressive power and, accordingly, neither Messiaen nor Sorabji can truly be regarded as especially influential, despite the importance of their music (especially their writings for piano and organ which are arguably as vital to 20th century music as those of Liszt and Alkan to 19th).
Best,
Alistair