Two teaspoons of sugar is more than enough.
Actually, it's not a partitive construction. Partitives normally require a definite noun phrase, the kind with the determiner "the", as in Some of the sugar is missing, or a genitive pronoun, as in Some of his food was eaten. But in Two teaspoons of sugar, "sugar" is not a definite noun, so this is simply a noun phrase with teaspoons as head and of sugar as complement.
Partitive is intended to denote a part rather the whole thing. Some of his food is a partitive construction in that the partitive his food denotes a quantity and Some of his food denotes a subquantity of that quantity. We understand it to mean Some food from his larger quantity of food was eaten.
Measure phrases like the plural two teaspoons in your example can be conceptualised as referring to a single measure which can override the plural form in determining the form of the verb. Which is why singular "is" is fine.