Yeah, I think your thumb has enough movement to tag a sprint shifter without disrupting your grip, if the shift button is placed well. Though I don't think reaching forward with an index finger is that bad. SRAM made a big deal when their road groups first came out about the "sprint position," where you can pull the shifter lever back to the bar and shift by twisting your wrist a little. But I always found that much more awkward than they claimed, and more difficult to do in a sprint than than just tapping that big shift paddle with my index finger.