Big Steve, as for me, I couldn't resist taking the underexposed roll in for processing, if only on the chance there's something interesting revealed in the results! So, yeah, I'd take it in for sure, and to me the question is whether to ask for push processing or not. If your lab guy is knowledgeable, you might consult with him on that question.
You might ponder the shots on that roll, and think back to whether there are some high-contrast scenes there. If so, that argues against a processing push, but otherwise it might be worth the risk. Since XP tends to be a bit low on contrast, the push might even be beneficial for flat lighting.