Don't think I've ever seen a 15"... You could stack'em for a total 16", but it may not be designed for the added leverage (the one on the transom)... Maybe you could use a 7" standard plate on the transom, or have a reputable jackplate shop like Hydrodynamics or such to cut you a static 7" setback of the same profile as the 8" champ setback...
No doubt.... But that is still a lot of set back....He has been reading some I bet... There are some guys running that on a drag.... Low hp motor can work that far back, But that is way back there... Not the norm..