Does Void Winnower prevent casting spells for even alternative mana costs?
The only time the converted mana cost of a spell changes is if is has an {X} in the cost and the spell is currently being cast. So Sheer Drop will always have a CMC of 3 regardless of if the Awaken cost is paid, or if anything else changes the actual amount of mana paid.

202.3. The converted mana cost of an object is a number equal to the total amount of mana in its mana cost, regardless of color.
202.1. A card’s mana cost is indicated by mana symbols near the top of the card. (See rule 107.4.) On most cards, these symbols are printed in the upper right corner. Some cards from the Future Sight set have alternate frames in which the mana symbols appear to the left of the illustration.