Man.... I'm so conflicted by these statements....
Ok, on the one hand, yes you get "more" bitterness at higher pH. BUT, the texture changes, too. Higher pH leads to coarser bitterness perception without necessarily being more IBUs. Though, the theory of more hop utilization at higher pH is sound. In fact, if you could take the boil all the way up to 8 pH, you could get 90% utilization.
...But, it'd be a viscous, harsh mess.
The reality is that some acidity smooths bitterness. A couple of tenths can make the bitterness seem stronger, but it depends on a lot of factors, like CoHumulone level, yeast pitch rate and a long list of others.
Kettle and mash pH drop because of calcium phosphate precipitation. The more Calcium, the easier the precipitation. The two important results of this lower kettle pH is better protein breaks, as well as shorter yeast lag times.
The first time I heard of a correlation between MASH pH and hop utilization was from the Co-Author of the water book, Colin Cominsky. His water is very different than mine (I'm also a commercial brewer). My water has calcium levels of 70 to over 100 ppm. My IPA mashes range from 5.2 to 5.4 and there is no change in measured IBUs.
Interestingly, Tom Shellhammer of Oregon State gave a presentation on IBU research they did. He found that tannins from grain and hop plant matter played important roles in both bitterness perception and measured IBUs. It turns out that up to 30% of measured IBUs comes from the plant matter, not the alpha acids. He showed that unhopped beer can have up to 4 IBUs just from grain tannin. Most importantly, he showed that CoHumulone is the fastest and most complete of the acids to isomerize. It also breaks down in normal boils and in packaged beer.
So, IMO, there is some creative interpretation about achieving isomerization, which is good. If it makes your beer taste better, by all means do it! Yet, as a real measurement, the research is incomplete. One thing is for certain, there isn't a formula to calculate it with.
