You make an interesting point, and what makes it very interesting is that it's testable: are native speakers worse spellers than non-native speakers? And which words do each group have the most difficulty spelling?
It doesn't seem likely that non-native speakers are better general spellers than native speakers; but it's possible that some words are especially difficult for native speakers and not for non-native ones.
Are you aware of existing studies on that subject?
Aye, there have been numerous investigations into it. Among linguists, it seems to be basically taken as a given that homophones, particularly when compared to less skilled native readers, are going to be easier for the non-native, because of acquisition methods. I should've prefaced that with the disclaimer that I am not a linguist, but I have worked in the past with linguists on language-related work, both in L1 (native) and L2 (non-native) contexts. Here are a couple of references I was given on this specific subject:
1. Van Orden, Guy C. (1987). A ROWS is a ROSE: Spelling, sound and reading.
2. Binder, K. & Borecki, C. (2007). The use of phonological, orthographic, and contextual information during reading.
3. Ota, M., Hartsuiker, R. J., & Haywood, S. L. (2010). Is a FAN always FUN?
It doesn't seem likely that non-native speakers are better general spellers than native speakers; but it's possible that some words are especially difficult for native speakers and not for non-native ones.
Are you aware of existing studies on that subject?