I'm sure Paul will answer your question, but here is a neutral opinion. I do run soft compound tires on the street for a long time.
In theory, soft compound tires will have a shorter life than stock rubber. But what will happen is the tire becoming harder. Not interesting anymore for autocross or track, but still liveable on the street if you could live with a little slippery tire.
Also, if you do normal mileage, these tires do not have much tread and will wear quicker.
In my case, if the tire still have some tread left after 5 years, I don't expect to keep them much longer.
What others have found about soft compound tires is they tend to grab all the little rocks on the road and throw them into your wheelwell. I don't care. It's the trade for having sticky tires that hang so well into curves. But for the majority of x 1/9 owners, it seems to be too much so they tend to stick to cheap-chinese tires that are becoming more available, and sometime do a good job for a great price.
So it's your choice. If you are race-oriented and are willing to do some compromise, R-compound tires are available easily in 13" sizes. If you want a more relax driving into your collector car, maybe stick to another option.
