I agree that the attack vector is still the same. I see the risks increasing if this is capable of calling out, however. You're absolutely correct that malware authors want to maximize both stealth and profit - accordingly here are the risks I see:
1. Communications are on HTTP port 80. This is unlikely to ever be filtered by any firewall software (unlikely as in...never). It's also always going to be permitted outbound through proxies. It's just one of those protocols that no one can live without. So the trojan has an unimpeded communication channel. Also, the format of the HTTP protocol is so loose that almost anything can be put in an HTTP payload. Sounds pretty stealthy to me.
2. If the web server on the other end is being written using "standard" technologies like php, then as long as it works, the "capital" involved in building it and the communication protocols to this malware is invested. It makes sense to use it (from their POV). Also, the comms protocol is probably generic, not OS X specific. Although we haven't seen it here (for obvious reasons), I wouldn't be surprised if there are Windows variants of this malware making the rounds. Especially since the first version of this trojan used badly designed "Windows-like" imagery. It sounds like the start of an infrastructure for a botnet C2 channel.
3. If the trojan is making authenticated logins to a server, then it can (via its HTTP tunnel) pass just about any traffic. It's possible for the server's owner to simply leave commands on the server for each infected host to retrieve and execute after login (a la the Browser Exploitation Framework). If a person is logged into the server, the HTTP tunnel could be used to establish a reverse shell and execute commands in real-time.
In many ways, this reminds me of the early days of the Zeus botnet (circa 2007), when the developers were testing its capabilities, before the big explosion of infected PCs in 2009. In this case, of course, we appear to have the advantage of not being Windows...which should prevent fully automated installation and privilege escalation - which is a good thing!
However, considering how many new threads are still showing up on the boards daily, plus the fact that only some percentage of people who install this thing are going to show up and ask questions...it's impossible to estimate how many infected Macs are out there...could be a few dozen or a few thousand. And if the id numbers in the login are somehow being assigned linearly...then based on Thomas' packet capture there are at least 37000 of them. That's profit for a bot-farmer. Hopefully one of these callbacks doesn't result in a keylogger downloading...