A reason for thinking that justification requires perfect obedience is from Anselmian perfect being theology. Perfect being theology starts with the premise that God is a perfect being and then from that premise it infers certain characteristics about the nature of God. The reasoning then for the Law requiring perfect obedience for justification would be as follows: God is the most perfect being and because he is the most perfect being he will have every property that is better to have rather than for Him to lack that property. It is better for God to have the property of requiring perfection in his law for justification rather than not. This is a strong intuition because of the fact that a perfect being would obligate perfection for justification rather than be satisfied with imperfection for justification. Therefore, it follows from this that God obligates perfection for justification. Furthermore, it also seems that it would be better for the most perfect being to have the highest quality of righteousness rather than lacking this property. The highest quality of righteousness would be such that it would require perfection for righteousness. God has the highest degree of righteousness and therefore requires perfection for righteousness.
This is for the paper I am writing for Active Obedience in my Holy Spirit Class @ WSCAL, so any feed back would be most appreciated. (Just so everyone knows this is a indirect argument for justification in Christ alone by faith alone because only Christ was perfect to earn justification in our place which is received by us through faith).