How did you bleed the system?
I just recently replaced the water pump (wasn't bad, but I was in there and it had 92000 miles on it), thermostat, and temp sensor and spent more time bleeding the system of air than replacing the parts. I tried bleeding following the procedures outlined in the service manual and by DDM with no luck, there was always an air bubble. I could fill the reservoir and I would have to wait hours for it to drain into the radiator to fill the system. I didn't have the patience for that and removed the upper radiator hose and filled from there to speed up the process. It could idle and rev without overheating, but once the car was driven and a load applied, it would overheat. There was still air in the system.
I was going to try the vacuum method, but before that I had an idea that was easier to try, using pressurized air to force the coolant into the system. I removed the vent hose from the expansion tank and put it in a bottle to catch any excess coolant, I also removed the vent hose from the top of the engine. I filled the expansion tank, set my compressor to 10-15 psi, and used an air nozzle on the expansion tank where the vent hose connects to force the coolant into the system, never letting the coolant get too low in the expansion tank. Once coolant was coming out of the top of the engine like a geyser with no sputtering or air, I reattached the vent hose to the top of the engine. I repeated the process of filling the tank with coolant and pressurizing until I was getting steady coolant out of the vent hose into the bottle.
After this, I let the car idle with the heat on max to get up to temp and make sure it didn't overheat with no load on it. I then took the car for a drive with the heat on max to get the car hot and again making sure it didn't overheat. After letting it cool completely, I topped off the expansion tank and haven't had a problem since. Using pressurized air to force coolant into the system was the only thing to work for me. The vacuum may have worked, but luckily I was able to bleed it before having to buy or make a vacuum bleeder.
From when I bought the car in 2013 up until changing the pump, thermostat, and sensor, my normal operating temperature ranged from 200 - 216 F, and based on the forums, this didn't seem out of the ordinary. However, since replacing the parts and bleeding the system its been between 187 - 205 F, with or without the heat or AC on. This makes me think there was an air bubble in the system from when I first bought the car. This experience has made me understand why there have been so many discussions about the cooling system on all the solstice/sky forums.
Trying to bleed it again can't hurt, but definitely look into making sure you have the air duct, guide, and deflector used to funnel air towards the intercooler and radiator. If you're talking about the two tubes that attach to the front fog light grills, those direct air into the wheel wells for the front brakes.
As for the vacuum, at idle in neutral or with the clutch in, my car holds about 21 in Hg. At idle with the AC on, about 19 in Hg. When you say, "idle (in gear)" is it an auto or manual and are you letting the car crawl at idle or is it at a stand still? I can try and replicate what you're doing and report back with my vacuum readings.
