I have only taken NBME 16 & 17 so I cannot speak to the difficulty of 15, sorry. I thought 16 & 17 were similar difficulty tbh but most people say they get a little bit harder each increased number.
The general consensus on prediction of USMLE is that the average of you NBME scores will be the closest to your actual score, so yes NBME 15 does count as a good predictor but the more NBMEs you take the better your prediction will be.