I think I have gotten to the bottom of the leak issue (for me).
Once you're sure you've got a good seal at the top and it can maintain vacuum, the problem appears to be the center pin.
In a brand new coil, the center pin at the bottom has a reasonably tight good center post. I've noticed as I've vaped it the heat from the coil affects the gasket between the center pin and the body of the coil assembly and the center pin become looser and the juice begins to get between the gasket and the center pin.
The advice to go to a thick juice helps but it just slows the process. Over time the gasket fails from repeated heating juice makes its way down between the gasket and the center pin. I've not seen anyone post this problem anywhere so far, and as far as I can tell, there is no solution so far - its inherent in the design of the replacement coil itself. I've observed this behaviour on all the coils I've bought, its not just one type of coil or batch.
Its ashame because its a great system if they could solve that one problem. I get about 5 days at between 10 and 14w before the leaking becomes an issue and if I clean up and ignore it, it progressively gets worse. I've tried removing the centerpost and cleaning everything and reseating it and it works a little longer but will go back to leaking as the process of the juice making its way down between the gasket and the coil body begins again.
I think they have to change to a more heat resistant gasket material and make the center pin a slightly wider diameter to solve this. There is no reason for the center pin to ever be removed so I don't know why they just don't press fit it with a better material than this cheap rubber.