The Expression Operators Reference illustrates that you can use operators like <= (actually has to be <=) and ! and ||. The implementation is unlikely to be the execution of the raw JavaScript so best stick to the documented operators and syntax only.
(I don't see ( and ) mentioned but they do appear in examples.)
See aura/aura-components/src/test/components/expressionTest/functions/functions.cmp for some example expressions that use entities like & and <.