I haven't done it by bus but I self drove some years ago and the E65 road from Split through Sibenik up to Rijeka isn't that fast - and then you still have to get Porec- Rijeka which I don't know at all. Also there are a lot of great places on the route (Trogir, Sibenik, Zadar etc) so it deserves having some time taken over it.
This site says that just Split - Rijeka by bus is 8 hours which I can fully believe.
http://www.croatiatraveller.com/central%20dalmatia/Split/Buses.htmI guess it all depends on what sort of holiday you want - if it is primarily sightseeing then I would have thought that the Split area must be used as the centre.