Never take it on any champion and i'll try to convince you of it now.
In the longest game i played of league this year (one hour in ranked) on vel'koz i played a good game and won the game. My damage dealt was 100,000.
Scorch did 1000. Of my total damage that is 1%. What i like about this game is how all the numbers fit perfectly (when rounded a little of course).
1 hour - 100,000 damage - 1000 damage scorch. The damage i could have been doing with gathering storm? *whistle*.
At 10 minutes you get 8 ap - vel'koz Q scales at 0.8 ratio. So every Q you hit at 10 mins does 6 more damage, that alone is better than scorch LOL. Scorch needs massive buffs to be viable imo, gathering storm is objectively the better choice all games will last at least 20 minutes and at that point scorch is totally irrelevant.