I looks like 1/2 MPG savings so far (on my test circuit through town), but will keep a close eye on it. A secondary mechanical linkage might be a wise purchase for me.
Since the idle stop had to be opened, and the idle mix increased I believe that the secondary main jets were providing a little gas before I blocked the secondaries. I wonder if the idle circuit gives better mixing. Anyhow, the smooth idle and smooth low speed around town alone made this a worthwhile experiment.
I just saw you're one of the other few members out here with a big old luxury car. I've since sold all of my V-8 RWD beasts :-( but still have a fondness for them.
Anywho, the idle circuit is designed especially to increase the mixing of the fuel, it mixes it with air before injecting it into the main flow, at least on all the carbs I've had apart (all motorcycle).
Also, just an idea, have you torn apart and cleaned out the carbs yet? It always amazes me how much this improves the driveability and power of my bikes. In most cases it's the difference between running and not haha.