What kind of RPM are people getting with the YS 45 with tuned pipe (Macs)?

I was getting about 16K without anything attached, and I have cut down the header to get about 17,300. Since this is on the bench, I figure it will unload at a higher RPM in the air. The specs say the practical range is 18,000. Should I not expect or try to get more than the 17,300 on the ground?