There have been a lot of threads on this, usually started by folks trying hard to lighten their packs. Please consider that a reliable WPB rain top is the best protection against hypothermia, or 'exposure.' No one dies from being soaked, they die form 'exposure.' If you're going to spend on anything, the rain top is a good choice.
I enjoy fooling around with fabric offerings from Quest and others, but I do not fool around where my rain top is concerned. If you can test and verify the DWR, vapor permeability and water resistance of a material, which I doubt, I suppose an MYOG rain top is OK, but it won't be as light as the welded seam ones now being made.
In addition to Richard Nisley's post, please read Alan Dixon's article here:
A less than 2 oz 'eVent' material was sold on this site, and Jerry's report is that it is not working in a rain jacket. There are several possibilities - here are some:
It may be the eVent cannot function as designed in such a light layup that none of the responsible manufacturers use.
It may be that BPL was 'mistaken' about the product.
Worse yet, evaluations of new WPBs on this site in the last couple years indicate that eVent is marketing its product to manufacturers under other names, and not taking direct responsibility, as Gore does, for the use of its product. Also, these same reviews found wide variations in the performance of eVent rain tops.
If more 'breathability' were a priority for me, I would buy an event jacket from the most reliable sources - example, the REI jacket - it is called 'Kimtah' or something like that. I printed out all the BPL articles and studied them repeatedly, and while they were not unanimous on all points, it was clear that some eVent products, being marketed as eVent or something else, were no better than GoreTex offerings, and sometimes worse in terms of breathability and/or water resistance. I do plan to take a look at the Patagonia M10 if and when the hype and price come down; but some posters have had problems with the durability of Patagonia DWR, once a staple, not to mention the DWR from Rab, also once thought to be consistently reliable.
No doubt all of this is frustrating, but wishing won't make it otherwise. Here's a list:
BPL articles on rains shells and pants: 2010-2013
Rain Pants Article - June 2012
2012 State of Market
Will's articles 2011
ID pullover 10 oz?
If you spend a lot of time studying these articles as I did, I really think you will agree with my conclusions. Alas, they are pretty inescapable. Because you may get only one chance to find out if your rain top is reliable in extreme weather, this is not an area for much experimentation, IMO.