I wasn't trying to say that 15 knots is 100% safe, but it's certainly not out of the realm of possibility. The US has spent a lot of time not just on overall submarine quieting, but also increasing the tactical speed (highest speed where a sub is still very quiet) of our subs.
The best way to know for sure would be to test it in multiplayer or in a custom scenario to test detection distance versus target speed.
The link below is to a pretty good intro into sound profile/noise levels for submarines.
http://www.armscontrol.ru/subs/snf/snf03221.htm
Mike