Wikipedia indicates that Wirrick was Sylvester's guitarist and co-writer by 1976, i.e. before even the first album by Sylvester. It seems that by the time of the 'Call Me' album, Wirrick was doing a whole lot of songwriting (together with Jeff Mehl) and production. So I would guess he indeed continued his guitar duty too.
P.S. Also of note that according to Discogs, the credits straight up list Wirrick as 'all instruments performed by' (apart from some additional stuff on individual songs), which is quite impressive.