I am not an expert in this area, but I have had some exposure to the issue. In the standard "model 5305" type SEP and SARSEP documents that are provided by banks, insurance companies, brokerage firms, etc. I believe that the 15% limit applies on an individual basis.
Several years ago, our firm had an individually designed SEP document that we would use in the case of a plan sponsor that had previously sponsered a DB plan. (Since you are prohibited from using a model SEP in that case).
It seems that as long as you stayed within the 404, and 415 guidelines, you could write a custom SEP or SARSEP that would allow you to exceed 15% on an individual basis and get the full 25%.