I suspect that the addition of Leghorn didn't help with aggressiveness issues. Barred Rocks are an amazingly stable, calm, relaxed bird, and the Black Stars I've had have seemed to have more of a Rock temperament, although significantly vinegary. I think that the Rock genetics make a huge difference in winter laying and temperament.
I would suspect that a Red Sex Link resulting from a White Rock hen might have a better temperament than the more common variations.
I think you're probably right. Ideal Poultry has a Red Sex Link that they market under the name "Brown Sex Link," which is an RIR roo x WR hen. I've had some Red Sex Links from Ideal, but never that particular cross and I suspect that the temperament would be similar to that of the Black Sex Links (same 2 breeds used, just different color variety of Plymouth Rock). It would be interesting to know. If any members out there are reading this and have experience with this particular Red Sex Link, maybe you can let us know.