"is a slightly sloppy forestay actually a real performance issue upwind if the sail is cut for it?"
A little while back, I tried to get an answer to this on Boat Design.net. I never did get a really satisfactory response. My own conclusion (for my F31) is that a taught forestay, everything else being equal, is better-- but that the cost of retrofitting running backs(in operational complexity, windage, weight of structural reinforcement, and $$) of getting a really straight forestay would be too high.
I decided that it is more promising in terms of overall function (but probably no cheaper) to focus efforts on reducing weight and windage, and on optimizing foils.