Every single report I've seen is that these exploits use 3.41 specific memory addresses and therefor will not work on any other version of the firmware.
I know there have been multiple reports of failure with PSJailbreak (and this is just a modified clone, after all) so no, you can't stay on 3.15 and have this work, at least not for the time being. That may well change in the future, but who knows, the security hole being used might not even exist (at least not in the same form) in 3.15.
It seems much more likely people will figure out how to get something OtherOS back via homebrew, but with full access to hardware this time since we can patch lvl2 and lvl1 with a combination of this exploit and the work done by GeoHot now. That'd be in 3.41 or higher. Really doubt people will worry about re-writing the exploit (which may not be easy) to add support for 3.15 directly.