Yes it is probably slightly weaker. It would be weaker if there are collisions in the 1-16 character password space for sha1 (assuming a 16 character limit on the password).
My point was that if you already have a lot hashed passwords then you need a way to transition to salted ones and this was a method to do that.
My point was that if you already have a lot hashed passwords then you need a way to transition to salted ones and this was a method to do that.