I wasn't trying to say that you could use the Grob to beat players significantly stronger than you, nor am I saying that if you follow the plans in the opening you will win no matter the rating. I'm merely asking if you should fundamentally change the plans of your opening based on your opponent's strength?
Basically you are asking a "how long is a piece of string" question and I don't think any answer I or anyone gives you will match up to your expectations.
Your opponents strength is only one aspect to consider before a game. When you rock up to play and you find your opening choice going sideways then to answer your question, yes you should re-evaluate your plan
But before a game? If you have every confidence that the opening you play (and by extension its accompanying plan) will work even against a higher rated opponent then why would you can the opening (and its plan)? Really, why would you? so....no
You asking a question based upon absolutes and until you realize that chess is not about such things maybe you might come to grips with it
There is a straight-forward refutation of the Grob, and you vcan find it vone many databases.
And how many of your average players will spend their time studying how to beat such an off-beat opening as the Grob?
anyone who has lost to you playing 1.g4 basically. we live in an age that you cant play outright refuted openings, before someone looks up what you play.