On what basis have you concluded that without filtration you are only getting 0.5 grade? Without filtration of any kind most papers and certainly Ilford will give you grade 2.

As you say that your developer is fine and presumably your exposure is correct and paper is fresh, I wonder if your conclusions from your step wedge is wrong or is it a question of a different set of perceptions about what each grade looks like.

There is no way that unfiltered MG paper should be grade 0.5. A grade 0.5 print from a properly exposed neg should be very soft and grey looking and without any real blacks and yet you say that the blacks are OK?

I am very puzzled