Speed Demon is supposed to have super speed, right. Then why is it that most other speedsters in the marvel universe can run circles around him? He can run at about 175 mph. QuickSilver can dust him at his original speed of 770 and now he can go  at least Mach 4 (3,080 miles per hour),  Even Whiz Kid, an Initiative recruit can allegedley travel at 30,000 mph. 

 How can he even be considered a threat to anyone or even hope to pull off a decent heist with these guys running around? If his powers come from the wellspring why isn't he faster?

Because 175 mph is fast enough to do some amazing stuff but as they introduce new characters they keep going faster and therefore have to upgrade the characters they feel should be at the forefront like Quicksilver. Speed is even faster able to traverse the whole world in the time it takes his brother Wiccan to teleport.

I'd say he seems to be a fair bit faster now.

As of Superior Spider-Man #1, he's "faster than spider-sense" and able to land "thousands of blows in a fraction of a second". So there's that.