Does the manufacturer's label or instruction sheet say "15 amp circuit" or does is say "minimum 15 amp circuit". There's a big difference. If it says minimum some where, then you're okay with the 20 amp protected circuit. If it says 15 amp only, then that is what you must use. You see, manufacturers lables and instruction sheets are enforcable by the inspector, and the installing electrician is also bound by them. See NEC §110.3(B)
110.3 Examination, Identification, Installation, and Use of Equipment.
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.