Sorry to disappoint, but I havn't actually done a whole slotcar casting. I use the RTV silicon rubber for all sorts of applications in my everyday work- there are lots of variations in curing time and durability depending on the particular type of catalyst you get. Some catalysts give a very flexible mould that is great when there are deep undercuts involved, but they tend to embrittle and are not so good for multiple use. For castings I use various grades of plaster. Not good for slot car shells.
But I have found Tirantis to be very helpful- they publish a lot of technical guides. So if you phone up and give specifications of what you want, you'll probably get the right advice. You just need a high impact strength resin that will take good surface detail.