>> If you want a highly-breathable windshirt that is lightweight, the choices are very limited at this time. In the lightweight category, I think the only currently available options are:
"Faux-dini" eBay special (very inexpensive and lightweight, but lacks hood and sleeves are short)<<
I must be missing something. The Arcteryx is 4.4 ounces with no hood. The Marmot Trail Wind Hoody - with zipper chest pocket, drawstring hem, and drawstring hood adjusters is 4.9 ounces. I just weighed mine (Men's Medium). It is extremely breathable.
All of the running companies make versions of ultra-light unlined wind breaker hoodies. Brooks makes one (their LSD jacket) that is 4.3 ounces (with no adjusters on the hood).
There are tons of options in this category. These can often be found for under $50.