You make a mistake (albeit one that is not uncommon): you conflate breach of contract with breach of the law.
Breaching a contract is not unlawful, it simply subjects you to whatever liability you contractually agreed to incur (whether expressly or otherwise). By way of example, if I agree to sell you 100 widgets at $100 per piece, but then I find another buyer at $1,000 per piece, I can -- not unlawfully -- choose to not sell to you, with the knowledge and understanding that I am subjecting myself to certain liability to you (which will vary by contract). I will not become a suspect in a criminal investigation simply because I decided to not sell to you.
In short, one can remain a "very lovely great" person even if they decide to breach a contract.
[This is not to be construed as legal advice, for which you should seek a competent attorney.]
The idea of an "efficient breach" being morally acceptable is questionable at best. It's a new-age theory of law that takes a macro efficiency approach.
There are many problems with the theory:
1. In practice, the theory is wrong because it doesn't account for legal fees. The defendant will never be made whole.
2. There are cases where the efficiency itself derives from problem #1. A party might breach because it knows that it would be inefficient for the offended party to sue.
3. The theory doesn't account for cases (including the instant one) where a party breaches because it thinks the offended party won't learn of the breach.
4. The theory doesn't account for cases (including the instant one) where a party enters into the contract with intention to breach.
5. The theory doesn't account for the break-down in the moral fabric of a society resulting from people's inability to trust one another even with the presence of a binding contract.
6. The theory doesn't account for the emotional harm experienced by a party who has had his trust betrayed.
7. The theory is incompatible with the ideal of דובר אמת בלבבו.
So you as well make a mistake (albeit one that is not uncommon): you conflate lawfulness with morality, and therefor fail to offer a complete treatment of the OP's dilemma.
In short, one can remain a "very lawful" person if they decide to breach a contract, but not a very "lovely and great" one.