The Opti apps for Collatz are linked on their main page. However, you don't really need them, since the stock apps are the same as the opti apps. Only reason to bother is that you'll have a pre-made app_info file to let you run two WUs at a time (not worth it since Slicker upped the size of the WUs) or raise the CPU priority (which makes little to no difference due to the way Collatz is coded). Long story short: probably not worth the effort.
Ok, thanks Maxwell.
I'll leave Collatz as is and cross fingers dnetc comes back soon.