You've got a fairly large ski with 130HP, personally I think 50 is a little optimistic. However you should have plenty of torque to pull a tube. Most states require a driver, an observer and room on the ski for the tube rider. I've tried it and it's almost more trouble than it's worth, have you considered a SeaDoo boat?
Scratch that, I mis-read your post, I thought it was a GTI, so yes 50MPH for a GTX is very realistic, maybe even mid-50's.
Lou