Jonathan, I am not qualified to answer that question but I can tell you a story that my rebbi, Rav Gustman ZT'L, told us.

Rav Gustman ZT'L related that when he was a young man, he asked the Chafetz Chayim whether he should spend time learning Rashbam's mathematical calculations in Perek Beis Kor (Bava Basra 101). He pointed out that in any case he would not remember the details of the calculations, and it is the end product that is important - so why learn them in the first place?

The Chafetz Chayim answered that what one learns is never totally "erased" from the mind. Forgetfulness is like a membrane that "covers over" the memory, but the memory is still there. What one learns becomes "his" and will remain with him in the World to Come, even if he forgets it in this world. The Chafetz Chayim therefore advised him that he should learn the calculations once, so that he may acquire that portion of the Torah, even if he is not able to retain them.

I'm not sure if that applies to the case at hand, but I am pretty confident that you will not be punished for Bitul Torah for spending the time learning Rashi!

Best wishes,

Mordecai Kornfeld