calzone that's a bit of a meaty subject.
Firstly, you are barred from being a governor (other than a staff governor) if you work at a school for more than 500 hours per year.
Secondly, there should never be a situation where a Governor (even with teaching experience and knowledge) is going on learning walks "making judgements and critiquing guided reading, phonics and teaching". That is an operational matter and even if they have knowledge they shouldn't be using it there!
What learning walks can be used for is for the Head Teacher/ SLT teacher who takes them around, to point out the key components of the guided reading they see, or to chat through the elements of teaching and how they come together to form a package of education.
When we talk about teacher performance, we are only ever given anonymised data and proportional analysis (e.g. 93% of lessons observed were good or better, up 8% from the same period last year). Same with pupil progress. In fact the one time we were accidentally given data with pupil names on it, within 10 seconds of it being passed around, one of us said 'err, should we have the pupil names on this?' and the whole lot was back in the Headteacher's hands.
As for power, we only have power within the constitution. There really isn't much we can do. We can't tell the HT how to run the school.
With the new constitutional framework schools can recruit governors based on skillset, too.