What is the contrastive phrase of "little to no"?
If you are not satisfied with substantial ( I owe this to Oldbag) may I then suggest second-to-none; second to none. An example: wild honey has little to no benefit for chest allergy but Frankincense herbal extract is second to none for curing it.
You can say, "...a substantial effect", "...a measurable effect", "...an obvious effect" - any idioms that might be applicable would depend on the particulars of the situation. You might say, "'X' really turned it around."