They have voluminous arguments for Vishnu's superiority over other male deities, but if I recall correctly, they also claim Lakshmi's superiority over female deities.
Is there Srivaishnavite scripture that claims this?
Yes. Vedanta Desikan's Sri-stuti:
59 SRI STHUTHI SLOKAM 12
AasamsAram vithathamakhilam vAngmayam yadhvibhUthi: yadhbhrUbhangAth kusumadhanusha: kimkarO MerudhanvA yasyAm nithyam nayana sathakai: yEkalakshyO MahEndhra: PadhmE thAsAm pariNathirasou bhAva lESai:Thvadheeyai:
Meaning according to Dr. M.NarasimhAchAry: Sarasvathy, Parvathy and Sachi are but modifications of minute particles of Your own graceful will.
Meaning according to Vaikunta Vaasi Villivalam NaarAyaNAcchAr Swamy: The Iswaryam of knowledge (VidhyA=Kalvi) that is spread all around the world is under the influence of Saraswathi DEvi, the consort of Brahma Devan; Lord SivA, the husband of Paarvathi is subservient to Manmathan fearing the raising of the eye brows of His consort. Indhran is wide awake with all his hundreds of eyes and is looking alertly for the hints form his Devi, SaamrAjya Lakshmi, IndhrANi .Oh MahA Lakshmi! The greatness (Mahimai) of Saraswathi, Parvathi and Indhran's wife are nothing but specks form the power of Your anugraha sankalpam (Volition, Sakthi).