The difference between the two sizes you mentioned is about 2.3%. So depending on your rpms at highway speed, the difference will be less than 100rpm. You're going to need 15s to make a dent in the highway rpm.
If you get the right backspace you could probably fit a 255/60R15 on the rear and a 215/65R15 on the front. Still if you do the math, that's only a 4% difference compared to what you have today. So for example if before you were cruising at 3000rpm, it's only dropping to about 2875 (a 225 rpm drop).
