I think that any lock who has played destro would agree that hard casting SF to keep up the buff just does not fit into the rotation well at all. So I was wondering, how could this be fixed?
Well, one solution I had was to modify the ISF talent. In addition to it's current effect, it now also makes SF an instant cast (or at the very least reduce the cast time further), but gives it a 12 second CD. This will make SF feel more a part of the fast paced destro rotation, instead of a long pause. This would, however need to change the Empowered Imp proc from making it instant, to resetting the CD. Thus, we still have instant SF from the Imp, cannot spam it, yet at the same time are not forced to make the dreaded 2+ second cast.
The second idea I had was to remove the buff functionality from SF altogether. Destro used to have a talent called pyroclasm that did the exact same thing, only it procced from Conflag crits. If they brought back the basic idea and tied the damage increase to conflag it wouldn't be a bad idea. We'd still cast SF on EI procs, but we'd never have to hard cast it.
Lastly, they could increase the duration, by... well, a lot. Enough so that it would be a statistical likelihood that one would get an EI proc before the buff fell off every time. If not, that's what Soulburn is for. This would probably be the best, and least complicated solution. It wouldn't completely eliminate the hard casts, but it would make them fewer and more far between, which is always a good thing.
The way I see it, the only way to "fix" the feel of destro is to remove that hard cast of SF, or at the very least, reduce the frequency at which said casts need to be made. To be honest, I would rather see them fix this than improve our damage right now, even though we took a bit of a hit in 4.2.