No, obviously you need to do the same or more total hashes as the real chain (it is a 51% attack...)
The "bad" part is that you can make your chain have more blocks while having the same start and end nTime.
Ok so it's a 51% attack which replaces the whole block chain.
You seem to be implying that the main negative effect is that the attacker gets more blocks over the period than he "deserves" from his hashing power. However, isn't the main effect that all the previous coinbase transactions are deleted and hence all transactions are declared invalid? This would effectively destroy the system and the response would be to either lock in the valid chain (preventing the attack) or start again with a new genesis block. Either solution would result in the attacker having wasted his hashing power.
Is this purely a destructive attack or can it be made profitable?
ByteCoin