Sure there is some perceivable flavor that comes from bittering hops but, for the most part it is minimal and it usually comes down to the "type" of bittering that matches well with the beer.
By type, I mean that some hops impart a very smooth, mellow bitterness at the lowest end of the spectrum (usually low alpha Noble) and at the other extreme, a rather harsh, biting bitterness comes out of some others.
A good deal of that is also an effect of you water chemistry too.
The IBU numbers are an average for the batch harvested and, calculations based on those numbers are for "ballpark" reference. So, IMO, Yes. There will be a slight difference but minimal and to discern the difference you would have to do a side by side.
There are 6 different main constituents that make up the bittering, flavor, and aroma that any particular hop will impart to a wort. Even subtle variances in those levels make for a "different" hop. Some of the constituents are extreme and others are very subtle.
For more about what it is in the hop that makes our beers tick I suggest you read George Fix - Principle of Brewing Science or Lee Jonson - Brew Chem 101.They both offer some pretty descriptive sections on the Lupulones (sp) and Humulones (sp). There are some other compounds in there too that effect the hop chracter (myrcene).
It's a good read but, can get kinda techy plus, given that we usually only see the tested Alpha numbers and only see the rest if we dig (even then they are avreage for the strain) deep enough.
Homebrew information has evolved a great deal in the 2 years I have been doing it, so it's very well possible soon we will receive the same respect that the breweries get and then the specific numbers will be divulged to us.