Originally Posted by brussum
Having experienced this same issue, I've always wondered about diluting the hops while diluting the fermentables. Assuming you hit target volume with a specific gravity that is too high, wouldn't adding additional water create a less hoppy beer? How do you mitigate that?
Sounds like the OP's boil off was a lot more than anticipated. So, diluting will have some effect on the IBU, but compared to the OG/FG being that far off, for me I'd rather dilute. Hop utilization will be different, but not a ridiculous amount.
If you've overshot you OG and hit your volume perfectly, then your efficiency was better than expected. Then you will have to make a decision. Like you said, if you add water, your diluting the bitterness as well as the wort at that point. That's always a 'game time' decision. How much water if any at all to add? (Depends on how many beers I've had while brewing that usually determines what happens then.)