You can open the aircraft TMD file using windows notepad.

Use the find function to locate the line Geometry/fuselage.

Under that heading you will see a line that says Mass = (some number in GRAMS) This is what the weight of the plane is currently when you load it in to fly.
The next line says RangeMassMax = (some number in GRAMS). This is the maximum the plane can be changed in the airplane editor.

Change this number to a higher one. Remember it's in grams here.

Then save the file. Start AFPD and edit the plane. Select Fusealge and change the weight anywhere up to the weight you noted in the file.
Save the airplane. Then check the weight in the first screen of the editor.

This doesn't seem to effect the physics just feels heavier and requires more power.