But, for the sake of conversation, back to Rob's fundamental question:
Are race-compound tires better in the rain that street tires made specifically to run in rain? I have wondered too whether some speed rated rain & snow-type street tires would not be better than even full-tread race tires in a deluge.

FWIW 2 of the most fun races I have ever had were when I had no rain tires, everyone else did, and a dry line appeared after a few laps. Ye-ha!