Del,
I'm sure quality of yew varies/varied considerably within regions and it would be very hard to compare different regions to each other with any definitive certainty. I agree and doubt medieval/early modern bowyers cared much at all about growth rings per inch instead of the overall suitability of yew staves. I would be interested to know your (or anyone's) thoughts concerning what's said in this source:
From The Art of Archery, c. 1515, anonymous (I like this source so much better than Toxiphilus, which is long-winded , specifically mentions Portuguese and Italian (Alpine)yew and contrasts them:
"First of all, bows can be made of any wood, but the best are of yew. Respecting this, Petrus de Crescens says, that yew is only useful for making bows and crossbows, and that there are two sorts of it, the white and the red. The white is called Portuguese yew, and it is usually soft and of open grain. And the more open the grain of a wood is, the softer it naturally is. On the other hand the redder yew is called Italian yew. This is found of straighter grain than any other, and has a sharper cast, and there is no comparison in the time it retains its strength. However, it is harder to work, and to string at first, and breaks more easily than the white Portuguese yew..."
One could interpret this statement in a lot of different ways. It doesn't necessarily mean that imported yew from the regions of Portugal or Italy was significantly better than local English yew sources. But it's interesting to note that these are the author's classification for yew in general (he doesn't even mention English yew or any other kinds), indicating at least a great deal of it was imported from these regions and that they were considered distinct from each other and had pros and cons. My thought is either the quality of yew from these regions was better in quality than English grown yew (as is commonly thought), OR as you said, the quantity of yew then grown in these regions was Much greater than any found native to England at that time (a lot has been planted since then).
(note: this thread might be better placed in the English Warbow thread)