Does this work transcend the artistry and involvement of the Opus Clavicembalisticum?
One could aruge that it does: I would be more inclined to say that it represents the directions in which Sorabji continued to move from OC through the much shorter Toccata II and on to Sonata V.
If you could compare it to any other works of Sorabji's that have been recorded, with what does it share the most qualities.
I wouldn't, really. It could be said to embrace, among other things, a more developed perspective on the characteristics of Toccata I, Sonata IV and OC but there is inevigtably more to it than just that.
How is this titanic piece structured?
The layout is described in the book Sorabji: A Critical Celebration (ed. Paul Rapoport) without which neither you nor anyone else with as much evident interest in Sorabji should really be without!
What did Sorabji think of the work after he finished it and moved on to other pieces?
He very rarely spoke or wrote about his own works.
When it is typeset (perhaps 10 years from now), I will definitely be curious to purchase a copy.
I sincerely trust that it won;t take that long!
I know I'm posing some broad questions, but I've never read anything constructive about this piece. Several people on this board have touted it as one of the hardest pieces ever, but I've never read a shred of elaboration.
This entire "hardest piece" business is - to me, at any rate, largely tiresome and usually far less constructively informative than some might assume it to be, for a whole raft of reasons of which I have written previously; in the case of this sonata, it would seem especially inappropriate and unhelpful for anyone who has not actually spent a very considerable time practising it even to speculate about such matters...
Best,
Alistair