Almost certainly not. I'm sure the 4:1 would be plenty. Raven is a 34' Van de Stadt steel sloop. Rig is only moderate size, masthead is 12.5 metres above deck. I only used the 8:1 as an illustration and will let my rigger tell me which size to use.
That looks pretty good to me. I'd need a couple of minor mods to your system but that's about all. Sure beats the hell out of having to shell out for all new back stay.
In general the Johnson system looks great but I can't find a distributor for their products in Australia. Price might also be a consideration. USD600.00 is about AUD780.00 plus shipping and taxes. Knowing the way Australian purveyors or marine products mark up their gear this will probably mean it will end up over AUD1000.00 which is getting a tad expensive for what I need. We shall see. I'll speak with my rigger (I do the work but they supply the parts and give plenty of free advice, great bunch of guys) and will report back in due course.
“Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life.” Terry Pratchett