That wouldn't be Bitcoin, but some other protocol/alt-coin. The Bitcoin protocol isn't only the longest chain; it's the longest chain that everyone agrees follows all the protocol rules, which includes not double spending coins and hash values which match all transactions (switched with malleability or not). A hash power advantage doesn't enable the circumvention of that.

Edit: I just got what you're saying. That would be a bit of a problem if the attacker could maintain 51% for a long period of time. Multiple conflicting transactions would break the attack, though, as there would be multiple orphan chains, but maintaining two chains of approximately equal length with one fork would mean users in the real world would need to wait to see what was the "real version of reality". I think most 51% risk comes from pooled miners, which in a stressful situation (like that of GHash.io) could migrate away, removing the hash power advantage before long.