Client has compensation greater than $200K as non-owner/employee of big corp and defers $10,500 to 401(k) plus a small matching contribution. This person also has consulting business with income of $200K, and wants to set up SEP or perhaps DC or DB Plan. Does he get the full 415 limit for this new Plan, despite participation in the 401(k) Plan?

Based on your facts, Client can set up a qualified plan and get another 415 limit. The 415 limits apply to controlled groups of companies, but not to unrelated businesses.

Agreed. Assuming the businesses aren't related/controlled/affiliated a separate 415 limit applies.