Since Axes loves his silly "40 DPS is a lot" calculation, here's the actual numbers for anyone else to achieve 40 DPS:
Base attack speed of .625. In order to reach 1.0 attacks per second (meaning you deal your AD as DPS), you need 60% bonus attack speed. For most champions, that is achieved from basic level ups. After this point, every point of AD increases your DPS by a parallel amount.
If you have practically ANY attack speed item giving 40% or more, you'll be dealing 125% of your AD as DPS. This puts Pickaxe (875 gold I believe) at 31.25 DPS. You don't have to buy anything else to get there, you just buy Pickaxe.
If you buy a Cloak of Agility (15% Critical Chance), you increase your DPS by 15% of your PREVIOUS calculation (not the base calculation.) This means that, with 100% increased attack speed, you deal 1.25 * 1.15 * your AD as DPS. Pickaxe does 36 DPS.
So with two VERY minor attack speed and critical chance increases, you can get your AD to DPS efficiency nearly to the level of Sunfire Cape, and the AD to DPS conversion has a lot better (and more) scaling functions to utilize, unlike Sunfire Cape which has, literally, only Magic Penetration.
So, yes, Sunfire Cape isn't bad. Early game. Once you get past a certain point, though, it is less efficient than practically any other stat. (Heck, I believe, with the scalings, even Atma's Impaler provides a better DPS efficiency, and it has even ANOTHER scaling function.)