Flyboy Dave,
I agree with you that with most carbs, a SLIGHTLY rich idle setting will not cause in flight problems at FULL THROTTLE. The most common problem with a rich idle setting, in my experience, is at part throttle when the throttle is advanced after a period of low throttle operation. In this situation MOST engines with a rich idle setting will be prone to gag and possibly quit.

In the case of the GMS carb however, the idle setting is extremely sensitive and definately does effect full throttle performance if set too rich. If your engine was delivered with the idle setting too lean and you have richened it just enough to allow it to transition properly, then yours is not too rich and so wont exhibit the problem we are talking about.

There is no doubt that these engines will idle and even transition OK with a very rich idle setting. The downside however is that in this configuration, the idle needle will withdraw too far from the spray bar tube at full throttle causing (I believe) poor fuel draw and atomization. This in turn requires a VERY VERY rich main needle at full throttle. In my case richening the idle needle by 45 degrees requires the main needle to be opened by over a turn!!
The problem is NOT the fact that the IDLE mixture is rich, it is that this rich setting means that air/fuel flow of the carb is screwed up at full throttle due to the relative positions of the idle needle and spray bar tube. With an engine set up like this there is no doubt that it is very likely to quit unexpectedly in flight - mine certainly did.

Once the idle is set correctly, the engine behaves normally - the main needle can be leaned - it is progressive and predictable - the engine delivers more power and becomes reliable.

I have set up engines of all brands over many years and have never before experienced a carb where the idle setting has such a dramatic effect on the physics of the carb at full throttle. I stress again that it is NOT the idle mixture that is the problem it is the resultant change in the geometry of the carb in the full throttle position.
Most engines can be tuned perfectly for full throttle operation whatever the position of the idle setting. The GMS certainly falls down in this respect.

It seems that a number of others have now tried setting their carbs as I have suggested and found that it has worked for them as well.

You are correct of course in saying that engines tend to lean in flight and the MAIN needle needs to be set slightly rich to compensate for this. The IDLE needle however certainly DOES NOT need to be set rich for any reason. I can see no advantage with any engine under any circumstances for having a rich idle setting.