Is there such a thing? As in, an IMS 18g basket? Do you know the model exactly?
Personally, I use a VST 18g (it’s written on the basket, 18g), and I dose between 18g and 18.5g in it.
I don’t think there’s an adverse affect in overdosing or downdosing within reason. Of course, if you have a basket which is designed to take 18g and you dose 20g, you will quickly notice that the basket is overfilled and the downside is that it will touch the shower screen as soon as the coffee expands or as you lock the portafilter, resulting in a cracked/fractured puck and ultimately channelling.
If you downdose, you’ll end up having to grind finer in conjunction with the puck be so thing and ultimately resulting in puck fracturing/channeling.
Directly to your point: For example, in a VST 18g, it’s OK to dose 19g depending on the coffee. For example, with light roasts, I feel that I can dose more. For a darker roasted coffee, not quite as much, and I feel that for certain coffees, 18 is too much. Hence why, for VST at least, the basket nominal dose is 18g, but they do say you can vary the dose by +/- 1g.
For the same coffee, if you updose (within reason) you will have to coarsen the grind to keep to the same ratio. And the opposite is also true (grind finer for a lower dose).
As per difference in taste… Not a clue. My tastebuds never reported anything back in terms of meaningful difference. 😊 - You can always try it and check if yours do.