I like the idea of Alchemists being able to go beyond what a 5th level mage could do, Their magic research throw is still limited and their research is more time consuming and costly. Plus it isn’t trivial to get 3 levels of a specific proficiency. They still have to worry about a difficult research throw and their costs can really spiral out of control if they fail a few times.
You could add some potential side effects for failing by too much when inventing higher level potions akin to the experimentation rules. Basically, they’re messing with powers far beyond their ken. They can try (after all, alchemy at that level is doing things normally restricted to mages), but it could be disastrous.