[Opinions] I have to disagree ...
Though of course I didn't do a study of it and I only base this on two Teagans (actually a Teagan and a Tegan) plus a Keagan that I have known of.
Teagan and Tegan both said their names Tay-Gan, like Reagan. Keagan did also.
Before you criticize a man, walk a mile in his shoes. That way, when you criticize him, you're a mile away and you have his shoes!
Steve Martin