Probably in a very heinous case, where the case law already demonstrates that sentences are greater than five years...in that case it may very well survive a constitutional challenge.
Where the legislation on minimum sentences runs into problems is for cases that may have mitigating factors, because mandatory minimum sentences, of course, are a sort of one-size-fits-all solution, ignoring the reality that offenders and offences can be very different.
There is a presumption that minimum sentences, when they're passed, contemplate all cases, the best and the worst. Of course, even the wisest of men can't see all ends. So there will be cases that attract scrutiny. Everyone is quite aware of the case of Smickle, involving guns. There are cases that could attract constitutional scrutiny.