Nope, 153 is pretty average: http://www.pqiamerica.com/Feb2014/consolidated5w20ALL.html
I don't remember where I read the 20% figure, but a quick google found this link that backs up that number: http://www.bronsautomotive.com/oil-change-intervals/
"Synthetic blend oil contains only 20% Synthetic oil and 80% regular oil in each quart. I do not personally use this type of oil, but if you do, use the same intervals as regular oil."
Of course I'm sure that most good name brand blends are probably much higher than 20% though and maybe 20% isn't even the real minimum, oil marketing laws are so vague and hard to enforce.
For a 5w20, a VI of 153 seems on the high end for a conventional, no? Can you share where the information concerning what % constitutes entry into the world of blends can be found? I never heard this before. Not being sarcastic either.