I'm sure Dave can correct me if I'm wrong, but the difficulty with foiling up wind is an available power issue. As the boat gets faster, the apparent wind comes more forward. As the apparent wind comes forward, less power is available. Except in the case of the uber-efficient moth (with very low windage and extreme lightness) most cats and tri's are going to exhibit a pretty high aero drag profile and find it difficult to take full advantage of foils completely around the race course.

And yet this is what existing cats are doing with T foil rudders.

Some benefits are worth the price. Foiled Moths win championships.

Right, but only because they are not using positive lift foils forward. As you already pointed, negative lift rudder foils make more sense then pumping water ballast into the hull: same displacement with less mass. It does increase stability.

The big tris alternate between using their rudder foils to keep the bow up or to help lift the central hull out of the water, according to the conditions. I am sure that at least Hydroptere and Geant can do both things.

...and this cat too - take a look:

The point was that foiling Moth and SYZ have a lifting foil in front, and do not overload the rudder flaps with a terrible load, or try to exert too much control, causing "dive brakes". Another problem not obvious is that the sailing foiler going to weather may run through a puff of wind and lose effective lift, coming off the foils: This is a real nuisance. Foilers need steady trade winds.