RE: Welcome to Club SAITO !
Yes, you can use a 15x8 on the 1.25. I don't know yet about run times on 14oz, but I'm expecting around 10 minutes on mine as it breaks in. Some of this was answered in the RCV vs Saito 100 thread.
You have 2 other threads started asking these same questions. It is best to stick to just one for the answers you are looking for.