This is only the case if the wheels are the same width.
If you go from a 9" et20 to 9" et30, yes the new setup will move the whole wheel in by 10mm.
BUT if the wheels are a different width, that needs to be taken into account
So, let's say we go from a 9" et20 to an 8½" et 30.
-The new wheel is a ½" narrower or about 12mm.
-Half of the width is taken from the outside, half from in.
---If the wheels had the same et, going from the 9 to the 8½ would leave 6mm more room to the fender and 6mm more room to the strut.
-Now lets factor in the new et. a 15mm difference...
-Outside: -6mm - 15mm = -21...21mm more room to the fender
-Inside: -6mm + 15mm = 9mm less room to the strut.
So to get this hypothetical new wheel to sit under the fender like the original, we would need a 21mm spacer...
Does that make sence?
How wide are the new wheels?
Do you care much about the 'stance'?
I cant comment on the bolts. Different wheels have different pad thicknesses. All I can say for certain is that XFs use 12x1.5 bolts. The easiest way to figure out length is to pull on of the original wheels off and see how much of the lug sticks out of the back. Measure it or count the threads. Then put the same bolt on the new wheel and compare. If it is too long, by how much? If there's an additional 5mm, then the new bolts need to be 5mm. shorter.