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すら has the same meaning as さえ, but sounds more old fashioned/stiff. It can replace を, which is what it looks like it's doing in this sentence. For example:
If you read this, you would know.
If you just read this, you would know.
さえ shows a minimum example. In your example, it would be similar to the english "even".
I understand how you feel, but you have surpassed even me in power.
より would not make sense in this context. しのぐ is a verb and cannot come after より. Like the examples above, すら is replacing を, which would have marked the object of しのぐ.
Related Post: Does さえ mean “even” in the following sentence?