I'm not sure what the benefit is of building a stud. Sure, it would potentially stop drafts coming from the (possibly) colder garage into the roof void above the house, but you've already insulated that area. Moreover, roof spaces are supposed to be ventilated anyway to avoid damp/condensation issues, so presumably even if you build a stud partition the void above the accommodation will still be well ventilated.
If the house meets current building regs, the existing garage ceiling should be two layers of plasterboard (or single layer of suitable fireboard) + a plaster skim coat to help prevent direct ingress of smoke and flame. If not, then upgrading the garage ceiling should be the safety priority.
Heat loss is a function of the degree of insulation, the area and the difference in temperature between the two zones. I guess any heat which does escape through the insulation would be better retained in the roof void if the degree of ventilation is reduced by a stud wall, resulting in a reduction in the temperature difference between the house and the roof void. However, if the loft is otherwise well ventilated I can't see this making a major difference.
You might be better off spending the cost of a stud wall on further upgrades to the insulation above the house instead.
Other consideration: If you did build a stud wall dividing the spaces above the house and the garage, you should still provide access to both voids in the interests of maintenance and access for the fire brigade etc
