Collectablity is based on how badly people want to own a car. IMHO it seems that the Roush's are overpriced and the performance is adequate for the price. I don't think they will be as collectible as a Saleen or a Cobra just because they aren't as wisely recognized.
I know that Roush was courting MCA pretty heavily the last few years in an attempt to increase their exposure and their presence. Funny thing is that right as it seems that people were starting to accept the cars they pulled their MCA support.
I agree with Mark that the Roush stuff is overpriced (which really is the original purchaser's problem) but I think they use pretty good stuff on their cars.
How much more are they going to be worth is a very subjective question? It all depends on how bad the buyer wants one or how bad the seller needs to unload it?