You may well be working closer to the "Best Case" scenario, as opposed to "Nominal". Until the predicted Sg starts getting close to around 1.1, one might not notice any dramatic or obvious twist rate-dependent effects, such as oblong holes in the paper and/or really poor grouping. With an Sg in the 1.2 to 1.4 range, all that might happen is that the bullet behaves as if it has a lower BC than it should. It would be very easy not to notice increased windage without some definitive reference point. So if you've always been shooting the 75s out of an 8-twist barrel, they're not making oblong holes, and they're grouping well, why would you have any reason to believe the BC might not be all it could be? Nonetheless, you may not be getting the full intrinsic BC with an 8-twist barrel.
As it turns out, I suspect my numbers with the 75s out of a particular 8-twist barrel rifle will end up pretty close to yours based on QuickLoad predictions (I haven't started testing them yet in that rifle). I had thought about also trying the 80s in that setup, but Litz' Sg predictions for the 80s are even worse out of an 8-twist barrel. That is not surprising as it's a longer bullet. Nonetheless, the predicted Sgs made me pause about the notion of usaing them in the 8-twist barrel. I also have a bolt gun setup with a 26" barrel in which I am currently using the 75s at ~2900 fps, and it has a 7-twist barrel, so it would be the better candidate for which to load the 80s. One thing you might look into is that I'm not seeing a big difference in predicted windage between the 75s at 2900 fps and the 80s at just over 2800 fps. In other words, there just doesn't seem to be a big decrease in wind deflection with the 80s out of that specific rifle setup. I would need a longer barrel chambered with more freebore in order to really take advantage of the 80's higher BC. If you're in the same boat, it may be that running the 80s doesn't provide sufficient advantage to warrant potentially having to deal with the twist rate issue.