"Shouldn't shift be twice as hot as the parentheses?"

For offline analysis? No. Shift should be as hot as the SUM of both types of parentheses. In practice, both parentheses will be equal in count modulo some epsilon for unmatched parenthesis in strings and comments. Therefore, shift will be close to twice as either parenthesis.

For online analysis? No. Shifted characters can come into existence without being typed. For parenthesis, autocompletion is one way. Automatic bracket matching is another. There are many more, including template expansion, copy/paste, and several paredit operations.

Shift is used for many combinations besides just parentheses, such as that capital at the start of this sentence. It would likely be more than twice as much.