I think there are two questions here to distinguish. The first is whether Luke depended on Matthew or whether both used a common source in addition to Mark. Goodacre may well be right on this, tho I have not yet come to a firm decision. The other question is whether Matthew itself is based on an earlier document in addition to Mark....this issue is rather similar to the question of the independent status of E or P in Pentateuchal origins, i.e. if the "P" and "E" material instead represent successive "interpolations" into a base text, rather than originating as autonomous documents in their own right. Or was the sayings material for these "interpolations" into Mark (freely adapted by the author) taken directly from oral tradition or from a written intermediate medium? I do not feel that Goodacre really tackles this question in a satisfactory way. A parenetic collection of sayings is the kind of thing that Thomas represents, and I do not agree at all that it is a third century composition....the extant text is third or fourth century, but the book itself is older (the Greek fragments go back to AD 200, several of its noncanonical sayings are attested in the mid-second century, and the theological outlook is much more proto-gnostic than the developed gnosticism of the Nag Hammadi community), my suspicion would be the first half of the second century. I also do not think that the sayings are based on the canonical gospels but are different enough in form (in fact, in a much more oral form) to be independent from them. The other major problem which Goodacre does not discuss to my satisfaction is the stability of the text of Mark....as he uses canonical Mark as the basis of comparison with Matthew and Luke, yet agreements against Mark may plausibly represent attestations of an earlier version of Mark, just as Luke may be viewed as a revised version of Matthew.