Here are the speeds I got:
Wings Only
Left/Right (Walking): 17 mph
Up (Flying): 67 mph (may go higher if I had deeper water to test)
Down (Falling): 26 mph
Up-Left/Up-Right (Flying): 78 mph (again may go higher with more water)
Down-Left/Down-Right (Falling): 48 mph
Shell Only
Left/Right (Walking): 17 mph
Up (Swimming): 24 mph
Down (Falling): 36 mph
Up-Left/Up-Right (Swimming): 30 mph
Down-Left/Down-Right (Falling): 40 mph
Both
Left/Right (Walking): 17 mph
Up (Swimming): 24 mph
Down (Falling): 36 mph
Up-Left/Up-Right (Swimming): 30 mph
Down-Left/Down-Right (Falling): 40 mph
So the shell cancels all effects in any direction from the Wings and though it makes you go faster when falling straight down, it makes you slower when moving down + left/right, up, or up + left/right.