How about any derived private key, which is not the root key? How serious a leak would that make? Does it compromise all the other private keys as well, if the attacker knows the chain code? (which I assumed is the same for both, private and public chains).
You should always assume that revealing a private key from a deterministic wallet will reveal all siblings. In Armory wallets, if private key x is revealed with the chaincode, all private keys >= X+1 are revealed.
But I don't spend much time worrying about this. We do not support or claim to support any use cases where private keys are intentionally revealed. And if your wallet is unintentionally compromised, they will all be revealed anyway.