oh the relative humanity of it...
I wonder what your temperature compensation table looks like?
Also, has anything else changed with the car? 10/1 ratio doesn't seem like it should cause the car to bog badly, something else may be up. I would check everything and make sure you don't have a boost leak after the turbo. I am no pro but if your using the MAF and you loose air after it has been metered it could theoretically cause a rich condition.
That being said I have blown up two STI's so my name is mud.