Couple of comments based on what I have learned over the years.
I have appealed darn near every year since 2008. Out of principal AND it requires the CAD to give you the comps and methodology used for the appraisal. One year, I called one of the tax reduction aggregators and asked to pay for 1 hour of his time. He said he would make more money just processing more appeals, then proceeded to talk to me for 30 mins. He was friendly and informative.
First, many of the tax appeal aggregators have worked for the CAD in the past. They make money on volume. I have seen these guys roll in to the TCAD office with a dolly and 4 filing boxes full of properties to appeal. They are chummy with the folks in the office.
Second, the land value is not arbitrary. I forget the EXACT methodology, but it is loosely this. They subtract the old land value from the new appraised value to determine a possible "new" improvement valuation. If that new value is 25% (? cannot recall the number ?) higher than replacement value for the improvement/structure across a neighborhood, it triggers a re-appraisal of the land values in the area.
Third, commercial property valuations have always lagged in Travis County. In 2013, the Driskill Hotel and related adjacent properties were purchased by Hilton for $85mm. Hilton proceeded to add $8mm in improvements. The 2014 and 2015 TCAD valuations were less than $40mm. This is after a VERY public disclosure of the purchase price. Today, I see commercial property being slightly depressed, given we really do not know what is going to happen with "return to work."
Last, the formal appeals process is problematic. After you deal with an uninterested and angry member of TCAD, you can appeal to a volunteer panel. This volunteer panel is composed of 3 retired folks from the community, with little or no expertise in property valuations. In one of my appeals, I showed data to support my claim, noted functional obsolescence in part of my property, etc. One of the panel members said, "I do not understand what Mr. Boilerhorn was trying to say, so I will side with the appraisal office." No attempt to understand, etc. It did not impede my case, as the other 2 members understood. Incidentally, the CAD used "he appeals every year" to kick off their defense of their appraisal. Uh, that should not matter.