A slower car. Assuming it's a mild bolt-on car. Speed density doesn't like major mods. But, they run quite a bit stronger in stock or bolt-on form than mass air cars. Leave it til you need to do the swap.
If you have plans of putting a bigger cam in(ie. E-303 or larger) it is largely recomended to switch because the speed density can not handle the drop in vacuum associated with a larger cam, very well, it causes idling issues along with a various array of problems. You can put better heads and a better intake and better exhaust as long as you keep the stock cam you should be safe. If you do a search on here you should be able to find more information as to why you should switch, I am actually in the process of purchasing everything myself to complete the conversion, I just ordered my conversion harness, and hopefully I will have the conversion doen this weekend.