The flywheel weight only changes the flywheel effect to smooth power delivery as the bike comes "on the pipe". It doesn't affect power, just power delivery. The GasGas 250cc engine will definitely lug. That is one of the great things about the GasGas engines. They were designed for cross country adn enduro, not MX.
The GasGas flywheel weights will install/uninstall in less than five minutes, if one has an impact wrench. (I prefer an air impact wrench.)
The flywheel weight is designed to replace the flywheel nut. (The flywheel weight has a nut milled into it's surface face, so all one has to do is to remove the ignition cover and then spin the flywheel weight off with the impact wrench. Then spin the flywheel nut on with the impact wrench.)
To install, just reverse the procedure. Paul Cantrell should still have the flywheel nut. If not, and one is desired, they are inexpensive, and easy to get.
I run the 10 oz flywheel weight on my 2006 DE300, as it is a bit "goosier" than the 250, down low in the rpms. I didn't run the flywheel weight on the 250 when I first set it up.
I hope this cleared up things up just a bit for you.
One more thing...The 2005 and later GasGas bikes really cut the tight trails easier than the 2002 through 2004. They do this without gaining headshake.
Happy Trails!