Key iteration count while using random keyfile larger than master key
Key stretching is only used to make small-entropy keys less vulnerable to brute force attacks. If it is (nearly) impossible to break the original key, than there's little sense in using a iteration count of more than 1.

If the input to the function is as big (in sense of entropy in bit) as the output, then an attacker could just attack the algorithm which did use the output and not the key derivation function itself.