Bold move. The plant looks like it's really thriving, so it should work out for you. Weird timing season-wise but the plant is in full leaf so why not.
One thing that's good to know is that Dorstenias which were started from cuttings will not develop the same kind of fat caudex as plants from seed. In other words, you are unlikely to convert a rooted cutting into a fat plant. Not saying it's impossible, just stating the general rule. That said, cutting the top off is definitely the best way to keep it from getting leggy.
Another thing.... D. gigas really demands a lot of light for good form, that probably above all else will make the biggest difference.
