I would say, "I would highly value..." I think it is a split infinitive or one of those things I never learned in school, but it sounds better to my ear.
I'm no flipping expert but I think that you want to keep "would value"...the two verbs together if you wanted to avoid a split infinitive. On the other hand, I believe that people should use the words in the way that makes the most sense and sounds right. To me, the "wrong way" is better. I would say "would highly value".
In my opinion, splitting up the two verbs is a way to place more emphasis on what you are saying. And IMHO splitting the verbs gives added emphasis to the word "highly". Crap, now I have to go look this up.