1. For a similar idea in Rashbam, see his commentary to Numbers 5:21 and note 21 there.
Rashbam’s explanation is similar but not identical to Rashi’s. Rashi says that the curses that fall upon the Israelites will become examples of terrible curses; people will use them as frames of reference when describing other curses. When something bad happens to someone else, onlookers will say, “The curses that we now see are like those famous curses of the Israelites.” Rashbam explains differently: a person who wishes to invoke a curse on an enemy will use the Israelites as a paradigm.

2. This alternate explanation of the idea of our text is introduced with the words “וגם יש לומר – it is also possible to say.” This phrase, and the similar phrase, “גם יש לפרש – it is also possible to explain,” (e.g. commentary to Leviticus 26:26) appear only rarely in Rashbam’s Torah commentary. In my Leviticus volume I argued that these phrases ought to be seen as signs of interpolations. See commentary to Leviticus 21:3 (and note 3 there) and see also the other passages and references cited in that note.
Would Rashbam be likely to offer an explanation like this? On the one hand, the idea that the nations will see the suffering of the Jews as a sign of their impiety is the simple meaning of Deuteronomy 29:24, and Rashbam explains that verse accordingly, also referring to that explanation in his commentary to 9:25-28. On the other hand, would Rashbam be inclined to introduce this idea to a verse such as this where it is not required? He surely was aware that this argument – the sufferings of the Jews are a sign of their impiety – was standard Christian theology, beginning at the latest in the days of Augustine and continuing through the Middle Ages (and beyond).