Could it means 2.5 oz total of Columbus, meaning you shouldn't have added 1.5 oz more (of Centennial in your case) @60 min.
Sound like a typo to me. 150 IBU in an RIS? That's gonna be intense. 66, while it sounds high, is a decent balance and will mellow with age.
The way the recipe is written is such (paraphrasing a bit)
Ingredients:
Malt extract and grains and whatnot (etc.)
2.5 oz Columbus hops (bittering)
1 oz Columbus hops (flavor)
.5 oz Columbus hops (aroma)
Directions:
Steep grains.
Boil the solution. Take off boil and add fermentables (all of them).
Bring to boil and add bittering hops. Boil for 45 minutes.
After 45 minutes, add flavoring hops. Boil for another 10 minutes.
After the additional 10 minutes, add the aroma hops. Boil for 5 minutes. Flame out, chill, pitch, etc.
So, from what I'm gathering, that means:
2.5 oz Columbus @ 60 minutes
1 oz Columbus @ 15 minutes
.5 oz Columbus @ 5 minutes
All the hops were 14.4% AA. Again, I'm not too worried about the beer being too bitter, I just still can't figure out why the IBU's would be so off. The IBU calculator I tried is here:
http://www.rooftopbrew.net/ibu.php