Not sure where you get your info, but the Thunder series has an adjustable secondary Air Valve. It allows you to adjust the secondaries to come in earlier or later, with the simple twist of a screwdriver. Something a regular Performer AVS does not have the option for. This is also "Not" the off-road version. So way off with your analogy.
This is actually my go to carb in my SB cars. Easy to adjust, and just works.
Don't buy new. Look for the guy who doesn't have a clue about his new carb, and is letting it go for penny's on the dollar.