Soulless Scourge is considered as a magic spell more than a summoning spell. You will notice that summoning spells has the white bar which shows the duration above the units.
I think his question was more asking along the lines of is there a bonus besides more damage for over cap skills like there is for summon skills.
Summon skills become gold units at 8/7 but other skills only gain damage or duration as far as I'm aware. It's a lot of bonus due to how scaling works but there is no added effect to the skill.