DM is usually full of Warwick Maths rejects, but that's okay, cuz you still get to do the same modules as Warwick Maths. You are just expected to do some CS modules, which aren't too bad, because the CS department is pretty world renowned too. Most DM students did do the STEP (but usually failed to get a grading), it's not needed for entry AFAIK. The maths you will learn at Warwick (whether that be within the Maths Department or the CS department), will be mostly based off of STEP questions, so the STEP is really good practice, even if you don't end up sitting it in the end/don't end up getting a grade.
On a side note, you should probably consider applying for Data Science at Warwick, it is a newer course, and is far more mathematically rigorous than DM.