One more material here that I use sometimes. This uses a lot of artistic license, but sometimes you just want a glass material to appear 'glassy'. This is a cheat that does not use Fresnel and is not fully transparent. It's a nice cheater. ThinCheatGlass.zip

As well as the other suggestions, have you tried increasing the HSph subdivs (under the irradiance map rollout) to something like 80-150 to reduce the blotchiness?