It adds 0.33 each time. Click on a Schlongus and see the number on the "Sap:" change after you add a Production+ pill.
Nope, I just bought one, and it went from 1/s to 1.17/s
I looked at the code again and everything seems to be correct, except that I didn't update the description. Production+ adds another 0.1666666667 each time. Thank you for the tip. I'll correct that in the next patch.