Push or pull can for E6 can also be done at RGB. Cost if memory serves is 50% more irregardless of 1 or more stops... I think they can push 3 max and pull 1.
Suggest people wanting to push slides should check with some bracketing. There is some evidence suggesting that 400 to 800 ie pushing one would lead to underexposure. Some compensation is needed. Try with 1/3 or 1/2 stop more exposure. Bracket to check what factor to use for your camera and way you are metering. No free lunches when pushing.