Post-quantum hybrid encryption

Assume the following simplified protocol:

• seed = curve25519SharedKey(theirPublicKey, myPrivateKey)
• sharedKey = scrypt(seed, salt, sufficientlyHardWorkFactors...)
• cipherText = aes256(sharedKey, clearText)

Does using a slow-hash on the output of an asymmetric key-exchange offer any more protection in a quantum-computing era? I know that quantum computing is supposed to make breaking diffie-hellman easy... what I don't know is exactly how easy. Is the algorithm (greatly) diminished, or completely broken?

No. $\:$ A quantum attacker would simply find seed even if you didn't use "a slow-hash on" it.
The algorithm will still be completely broken by quantum attacks. $\:$ Furthermore, your simplified