You mentioned that there isn't much difference between 1/2 throttle and full throttle.
On most engines this is normal, most of the throttle control is in the low end.
If you have a computer radio you may be able to set a non-linear throttle curve which for one click on the transmitter will move the throttle arm less on the low end and more as you get into the high end.
If you do not have this feature you can do it to some degree mechanically.
Set your end points to max and move your linkage on the servo arm and the throttle arm so you can use all of the available travel.
Next adjust the way the servo arm sits on the servo so the low end is almost straight forward or back depending on your plane.
Each click on the transmitter will move the servo a specific number of degrees, the distance the push rod moves is determined by the length of the servo arm and where the arm is in the arc. As the arc of servo arm approaches 90 degrees to the pushrod the amount of movement on the push rod per click is maximum. If you can get the throttle to fully open when the arm is just past 90 degrees this is the area where each click moves the push rod the most.
Hope this makes some sense, if you want to try to get some RPM change for each click on the throttle you might try messing with it.