You may only reduce the amount of ventilation when a vapor barrier having a transmission rate not exceeding 1 perm is installed on the warm side of the attic insulation. Paragraph 806.2 in the CABO Code talks about minimum free ventilating area. I believe R30 fiberglass insulation is approximately 9 inches thick. If I am correct, R30 insulation between 2x8 rafters will not allow proper ventilation. This does not meet code.
Warmer air can hold more moisture.
No problem with condensation there . Condensation is a problem of warm wet air being cooled below the dew point. If the house is air conditioned and is cooler than the outside, then you have a problem because the warm wet air will be cooling inside of the insulation. There is differences though. There is a much greater temperature differential with a glass of ice tea which is at 32 degrees F sitting in 85 degree air.