How bitter is that? Given that describing bitterness is pretty subjective, I think an objective form of measurement is probably the best way to answer that question. So, I'd say 45 IBUs is about twice as bitter as 23 IBUs.
Sorry, had to be a smart-ass.
I've got a couple assumptions I'm going to try on for size (you could post a recipe if you want to confirm or deny any of these). First, I assume you are pretty new to brewing if you are asking how this would effect your beer. Secondly, I assume that also means that you did a partial boil and brought it up to the full volume by adding water to the fermenter. Thirdly, I assume you did not thoroughly mix the wort and the top-off water.
If the predicted OG from the recipe was way lower than the 1.099 you measured, then you probably got an inaccurate reading from insufficiently mixed wort. In that case, if it is actually an average strength beer, the extra half ounce of hops will probably have a noticeable effect. It will still be beer. And it still has a fair chance of tasting good, too. But there will be more bitterness.
If you actually did brew a batch with a 1.099 OG, then (like HopZombie said) it is probably a good thing to add the extra hops to balance out the residual sweetness of the malt that the yeast can't finish off.