Not particularly strong. This is a classic case of a circumstantial argument for the identity of an early colonist, which (unsurprisingly) connects him to a mother (Margaret Campbell of Keithick) with demonstrable noble ancestry. I have just dug up the two articles by Charles G. Kurz (based on research of Thomas Garland Magruder, Jr.) which build the case for his identity and lay out some details on his alleged mother’s ancestry. For those who are interested and who haven’t seen the original articles, I’ll make them available here for download as pdfs:
These articles do make some effort to cite primary sources, but there is no systematic presentation of the strengths and admission of the weaknesses of Magruder’s claimed parentage. Kurz treads very lightly on the idea that these Magruders have nothing to do with the MacGregors, while still indulging in the old game of self-congratulatory worship of noble ancestors. Speaking of which, here’s something a little weird: the mixed-genre poem by Susan Tichy (poet, and Associate Professor of English at George Mason University) called “Heath IV,” a modern reflection on Magruder which of course preserves some of the old canards about his life and spurious relation to clan Gregor.