I think Designer is great. I often design icons though, where I would produce the same or similar image at a variety of resolutions.
It would be good on the Export persona if you could define a slice and also assign an output pixel dimension to it, e.g. 32x32, and then if the pixel preview would also reflect that would be great.
At the moment I realise that I could just duplicate the object in question and resize it to the required pixel dimension, but it would be better if I could have just the one item with different export profiles, duplicating it only for those resolutions that would benefit from some extra changes.