The GXE steel wheels are lighter than the Spec V rims, so you'll actually get a little increase in torque and acceleration. But you must get tires that will give you an over all same diameter and width as that of a Spec V tire, so that your speed guauge won't be off.
IF you have 195/60 15 tires on those rims, then they won't affect your speedo at all (or so little, you won't notice it). I plan to get rid of my 15" steelies I am using for the winter, and get a set of 15" aluminum rims off another model Sentra (like to 2.0SE).
My winters on my Spec V are Toyo Observe 195/60/15's...
That depends entirely on the manufacturer of the tire, as not all use the same construction or measure/calibration standards.
a 205 width tire, while it will also fit, it will actually make the car steer slower, because the sidewall will be bowed away from the rim more than a narrower tire. You want a skinny tire that is set to a higher pressure in the winter.