Fast and weight are not actually related. Makers of fast rods often overrate the rods they make, but that does not mean yours is. Personally, I have a very fast 8 wt. that has a higher ERN, but I still have an 8 wt. line on it. You need to carry more line to load it, but you can cast it a long way like that.
I would test it to see what it really is, because fast is not related to that. Then decide what you want to do with it as far as how far you need to cast it. If it turns out to be a 9 and you are casting in the short ranges, go get a 9. If it is actually a 7 I would stay at a 7 regardless of short casting ranges. If you are trying to hit 100' type zones you may want to go down a weight, provided it really is a 7. Test, then try a couple lines if you have a place you can do that. If not, test and make the decision then, but test it either way.
Over or underlining a rod without really knowing what it actually is (not what they claim it is, different things 99% of the time) is ill advised.