I fast tip shoot:
- use Area Light Inverse ^1 instead ^2 - will be no noise! You can put Area Light exactly size windows and still will be no noise. I know this type of light has not physical behaviour but apparently it works great with Kray. These renders from corona are based on that solution.
- also because these area lights has a bit different behaviour the setting of FG can be lower so you can try Final Gathering with stronger setup.