I agree that it would be a little cleaner, because it would express the intention better (the fact that you're simply giving out the same error). However, the type would be wrong:

Yeah, this is correct. This would be wrong:

    func map<U>(f: T -> U) -> Result<U> {
        switch self {
        case let .Value(value):
            return Result<U>.Value(Box(f(value.unbox)))
        case let .Error(error):
            // self is Result<T>, but the return type must be Result<U>
            return self
        }
    }

As for efficiency: like you mention, enum are value types. For this reason, they're passed by copy! So even returning self would return a copy of self, not the same instance :)