Yes, I do think it's conceivable that in relying upon a creative interpretation of what constitutes—I can't remember the exact language—grossly disproportionate, cruel and unusual punishment, even restricted to the facts of the case, you could find judges striking down laws and not rendering justice.
As I suggest, a way of narrowing this further could be by tying certain objective factors to making someone eligible for a lower sentence—things such as age, prior convictions, whether there is a commercial purpose involved, or threats, intimidation and the presence of weapons. This is used in other jurisdictions. They have the effect, I think, of constraining the judiciary appropriately and ensuring the intent of the legislature to ensure that severe punishment of an offence is upheld.