While I appreciate that a lens does not make a photographer good or bad, technique does, my question was not "Please tell me why my images are dull and lucklastre". I plan to buy a new lens in the coming weeks, I appreciate the look of the sigma 10-20 straight out of the camera however i do not like the look of the 18-55 IS STM images out of the camera, read as no processing. I get high levels of noise, poor contrast and colour, some of this may be technique but I dont think this is the case all of the time. I believe this is a factor of the lens design. Yes IS and STM are great and the images are sharp(ish) but thats it. So telling me my technique is at fault is useless for answering the question.
So back to the question will an L lens have noticeably improved optical characteristics such as colour and contrast over a non L ef-s lens such as an 17-55 IS USM or 15-85 IS USM? I am mainly considering the 24-105, 17-40 and 24-70 (all f4).