This might be a long post, so apologies in advance, and I'll be typing as I think, so it might be a bit foggy.
Ford has long positioned themselves as more of a people's car brand. They have worked hard for this, in terms of advertising strategy, so it would be unrealistic and contradictory to expect a couple of new models to change the perception that people have of Ford, and of the Focus in particular which is sold as a reasonably priced family hatch, 99% of which are basic diesel and petrol models. The RS will not transform the Focus as a model into a luxury purchase. To badge connoisseurs, the RS will always inspire respect, but to badge followers, the RS badge will never be enough to make them forget it is a Focus, a cheap family hatch. Many people of the kind that wouldn't be seen dead in last week's fashion, go out to be seen to be in the latest trendy spots, and make all their purchases based on how they will be judged by others would rather die than be seen in a Focus. To people who always go for "the best", which usually means the most expensive, whatever it is, whatever the price, the GT would be the target. So even in the Ford catalog, the Focus will be eclipsed. The Mustang will have an easier time seeing as it was a born a sports coupe and not evolved in a lab from a family car. The RS will have 5 doors, a generous boot and a fixed roof. To some, even RS owners, that's already a problem, so imagine how reluctant the badge following crowd could be.
What the RS can do is shine a new light on the model, by showing just how how far the car can go when given an injection of goodness. I actually think the Mk3 could have a lasting impact on many people's perception of the Focus, and I'm almost certain it will attract a lot of new customers that come from outside the typical RS and car enthusiast circles. Would someone who considers going for Merc or BMW also consider the RS? I think this is where deeply engrained cliches come into play. "German cars are more reliable", "Mercs cost a lot but last forever", "Fords depreciate badly",etc. Badge followers would probably not even consider the RS. But I believe the mk3 could be a turning point in terms of raising the profile of the RS brand. It will take time, and a lot will depend on reliability and other factors, but the potential is there.