Hopbursting / Late-hopping and bitterness during chilling

Does anyone know how much bitterness is actually extracted during chilling of the wort? I am working on a new APA recipe, and all of my hops are added during the last 15 minutes of the boil. According to BeerSmith, the IBUs should be about 41, however, I just tasted the wort, and it seemed very bitter to me...I just wonder if there is significant isomerization of alpha acids occurring between flame-out and some unknown temperature at which the isomerization stops occurring? I'm using an immersion chiller, and it probably takes 30-45 minutes to cool to pitching temps...

BeerSmith is saying that my "flame out hops" should be contributing 0.0 IBUs to the beer...but I guess I'm questioning the accuracy of that assumption. Perhaps the flame-out hops are actually contributing as much bitterness as they would if I added them 10 minutes prior to flame out and then used a counter-flow chiller to rapidly cool the wort within a minute or two of flame-out?

It will probably tame out during fermentation. There is isomerization down until about 175*F.

Thanks grathan. I just racked it to the secondary, but took a taste test before adding the dry hops. The harsh bitterness that I tasted right out of the kettle was gone. Any ideas about why? Do the yeast somehow separate some of the bitter components of the hops (cohumulone? Non-isomerized-alpha acids? Beta acids?) and then blow that up and trap it in the krausen? Is that the gnarly ring that I see around the top of the fermenter?

Anyway, besides being yeasty, I think this batch is going to be killer!  ;D

My wort always tastes more bitter than the fermented beer does later.  I think it's because no matter how finely you screen out the wort, it's got some very fresh hops in it that only scrub out and then crash out during ferm and after some time. 

Some of the iso-alpha-acids will actually stick to the yeast cells.  So, there is an actual decrease in IBUs during fermentation.  Ive never seen this quantified, anywhere.  However, given that it is an effect resulting from coating the yeast cells...you would expect it to be a function of yeast mass...and thus starting gravity. 

Second, the hop material increases the harshness of the bitter perception.  If you take a sample directly from the boil kettle with a lot of suspended hop matter and taste it, it will be quite harsh.  If you then let the hops settle out for 5 minutes, you will notice a much less harsh flavor. 

Further, the hop oils can be scrubbed out by the CO2 effervescence.  This doesn't decrease bitterness, but it can decrease the perception of it, because the total "hop character" has decreased. 

Finally, bitterness will drop out of the beer significantly over a few months.  From memory, its something like 50% per year.