Some have pointed out that the early studies showing high M26 in Castile suffered from small sample sizes. Further, more recent posts and agglomerations of studies show different numbers than what is quoted below. While Eupedia is usually full of pseudoscientific malarky, this post purports to have collected studies. And this website claims to have done the same, specific to Spain.
So, taken together, we get the following on our M26 I2a1a leaderboard:
(1) Sardinia (Island Isolated) ~37%
(2) Samnium (Mountainous Central Italy) ~10%
(3) Navarre (Mountainous Northern Spain) ~9%
(4) Sicily (Another Island) ~7%
(5) Basque Country (More Mountains) ~5%
(6) Aragon, Spain ~5%
(7) England, Ireland, and ALL W. European Islands whether Mediterranean, Atlantic, or Channel ~3%
We also have some allegations (no studies cited) that Andalusia (Spain) is in double digits.
From what I can tell, this is the picture that has emerged. Please cite links to studies if you have any.