I think it is route and cabin dependant. I think the sweet spot is WT+ to Club World, if you pick the right flight with a small WT+ cabin. I've found many other factors, but normally a single passenger on a booking helps - I've only flown four sectors in WT+ in last 2/3 years and been upgraded twice. Never J to F. I imagine this is harder due to the number of PAX in J, I'm too low down status order.