As fun as an instant cast Soulfire would it its not how it works. 1 stack does decrease the cast time of soulfire by 50% but 2 stack decrease cast time of 2 soulfires by 50% not 1 cast by 100%. As of right now molten core is really random. Sometimes its an instant cast ,sometimes its 10sec cast time, sometimes 2 sec and sometimes even instant (goes from a range of 20s-0s cast time from what I've seen). Demo right now isn't in a good spot either even if it would work perfectly , Blizz nerfed them far too much in 5.4! Soulfires are easily counter by cc-locks of enemy arena teams so your best way it to go for touch of chaos.