Improved shadow bolt was completely different from scorch. Scorch was more closely related to the shadow priests shadow weaving (9% increased fire damage vs 10% increased shadow damage). Both shadow weaving and scorch were debuffs that just had to be maintained on the target. Improved shadow bolt however worked completely different as I explained above. The more warlocks with improved shadowbolt you stacked, the stronger it became. Therefor it was extremely beneficial to go for a 'shadow setup' rather then a 'fire setup', and this is what top guilds did during sunwell.
I'm also sure rogues with dual glaves were very capable of beating 2.8k dps, I clearly remember seeing logs from both hunters and rogues beating 3k dps on terron gorefiend and brutallus.