Measure yourself before and after you try on your wife's size 8 Payless heels and get back to us with the difference. IIRC, you gain roughly 1/2 the heel-height.
I worn Elevator Shoes for about five months....before I stopped wearing them because I felt so goofy and fake. Those shoes increased my height by nearly 3". I prefer to be authentic with my image. No more contact lenses too, just glasses. I'm still insecure though about my thinning scalp.