Chuck and Zag both have it right...... this is the nature of pipes.

If you want it user friendly......easy throttle response...... it has to be a bit longer.

If you want all out performance in the air, you actually want to set it so it's short on the ground, which makes the engine a bear to needle (which is what you are seeing). The engine will surge and may only jump on the pipe after one of those surges. Then it will want to say there and not trickle off with throttle movement. In the air as it unloads, it will go to full song, and will tend to be well behaved now that it is within the tuned range of the pipe.

On a ducted fan setup (expecially the 7.5 engines) the later is typically the way they were set up for fan use to get every last bit out of them. There was little you would notice of a 'midrange' throttle setting.

Unless you absolutely need those last 500-800 rpm, lengthen the pipe up a bit. Let us know your results.