I think you have a serious backlash on X or Y (depends on which one you call X or Y). Best to measure that first. Once you have eliminated backlash you have to square the machine axes. I don't know why you have 1574.80315 (steps per?) and what you mean by 4000 / 2.54. The "steps per" value is a calculated value based on your lead screw pitch and microstepping. Without more detail about your machine it is not possible to say which values you should have. The problem of too fast rapids is easy to fix. set the value you are comfortable with.
Another thing, if you are using Mach3 it is a good idea to read the manual. It explains also how to tune your motors, measure and eliminate backlash and explains every item in the configuration menus and a whole lot of other things.