I like where you are going with the specification comparisons.
I found additional details for the certified external GPS in the ADS600-B installation manual
. In this manual, the GPS has SBAS/WAAS features which appear to be missing on the uncertified GPS. There is nothing about WAAS performance on the ADS600-EXP that I can see either.
I also don't know enough about TSO and performance numbers to speculate much more...
Oh - and the price is now $729 today. lol!