No single test type, such as breathability, will allow you to understand which is the BEST option for ultra-light backpacking. The problem is FAR more complex than what $50K of test equipment will answer.
You said, “BPL needs to flip the market upside down and come up with an objective breathability test for WPB fabrics”. Roger correctly answered that there are many “standard” breathability tests. Natick labs believe that the DPMC is the most telling of the standard breathability test options. $50K MIGHT cover the cost of building and executing a DMPC test of the UL options. This test will tell you that eVENT (ePTFE) is best and the hydrophilic PU coatings, favored by UL backpackers for their light weight are the worst.
To really understand the true relevance of breathability tests also requires an understanding of metabolic rates, perspiration production, and vapor permeability requirements.
If you are selecting a bivy or protective garment to wear while sleeping, you only need a WVTR of ~600 g/24h/m2. Any DMPC tested material is generally adequate for this environment.
If you are selecting a protective garment to wear while sitting around camp, you need a WVTR of ~1,300 g/24h/m2. Any DMPC tested material is generally adequate for this environment.
If you are selecting a protective garment to wear while walking around the city, you need a WVTR of ~2,500 g/24h/m2. The top 50% of the DMPC tested garments are generally adequate for this environment.
If you are selecting a protective garment to wear while backpacking in the mountains (6 – 8 METS), you need a WVTR of ~8,000 – 10,000 g/24h/m2. The most breathable option, eVent, only provides ~5,000 g/24h/m2.
To not have WPB garment breathability limit your work output potential, it would have to have to be air porous with a rating of approximately 400 CFM (loosely woven cotton shirt and pants). The most breathable WPB, which is truly rain proof, is .84 CFM, to put this requirement in perspective. Once air permeability gets beyond this general area they are no longer rain proof.
None of the standard breathability tests measure breathability in the rain. If you do a Vidybel (2 hour flex test in simulated rain) you will find out that the best and worst WPB options, from the breathability test, are reversed when it comes to leakage from light rain.
If you do a P. Salz test (1 hour in simulated rain without movement) you will find that the breathability of 2-layer eVent actually goes up, 3 layer eVent is only slightly reduced, and the hydrophilic PU coatings reduce their breathability to 19% of their dry value.
If you do a J.E. Ruckman test (1 -30 day simulated rain) you will find that the breathability of all garments varies dramatically based on temperature of the rain hitting them. You will discover that 5.6 mph wind combined with rain also further reduces the breathability of all WPB types. You will also discover that after 24 hours of simulated rain that eVent breathability drops approximately 10x and then stops breathing completely after 7 days. The hydrophilic PU has the same breathability as eVent after 24 hours and maintains this level of breathability through a 30 day test.
If you conduct ISO 6330 Wet-Flex tests you will find out that PU garments wear out and leak in the range of 100 - 250 hours of use. In contrast, both eVent and Gore-Tex last more 500 hours before they begin to leak.
No one test is adequate to evaluate WPB materials. Most people use WPB for lower MET activities and they periodically reactivate the DWR in a dryer; it works adequately for those environments. No WPB material is close to adequate for mountain backpacking in the rain. In other words, it is exactly as you postulated, "There just seemed to be a lot of things missing, unless the motto at BPL is all WPB suck which should be stated..."