What is described in the post won't force end-to-end encryption. It is more or less the same as using your current IM client over Tor: you still depend on a server which you may not control and communication between this server and your contact (directly or via your contact's server) may not be encrypted all the way. To be secure you would still need to use OTR, even if your contact also uses a Tor enabled IM client.

Otherwise if you want end-to-end over Tor communication, Ricochet works quite well :).