You can always plug in the GSU to see if you are getting the full rated RPM and EGT out of the turbine.
If the RPM's and EGT are there then the thrust should be there, unless you fly in sand and have altered the shape of the turbine blades.
Higher altitude will reduce power, I don't know that 700' will make a noticeable difference. Every field that I have ever flown at was between 600 and 675 feet.

OR, Just face it that you are getting good at flying that ship.