Quoting the report[1], "A boolean guard, g, is semantically equivalent to the pattern guard True <- g," which means the answer is "Yes". A boolean guard is equivalent to a pattern match. A predicate involving ==, however, introduces an Eq constraint that would not be required by pattern matching. For a properly equivalent guard, you need to write your predicates using pattern matching
isEven Even = True
isEven _ = False
to avoid the spurious Eq constraint.