It takes very little throttle opening to get my 454 to 3,000 RPM and I am running a moderately large impeller (A-cut). That impeller only absorbs arround 87 HP to spin at 3-grand.
3,500. aprox. 136HP
4,000. aprox. 202HP
5,000. aprox. 395HP
5,500. aprox. 525HP
Jets demand power on a non-linear curve that increases faster the faster you spin them.
At idle, my impeller demands 2.0HP to spin it.
At low RPM's, they are.
At 50 MPH, what RPM are you turning? As I recall, Wes's 21, with a 572 in it, ran on the top at about 76 MPH. Hopefully Wes will chime in here. Every jet boat I've ever seen sounds like it working hard at 50, except for maybe a 19' TPR Shadow or something similar.