Yes, technically Joe is right. The flywheel weight won't add any low end. It will however make the low end more tractable, and make it very difficult to stall the engine. If crawling through tight single track is the objective, a flywheel weight would be a benefit in it's own right, even if the effect is quite different from switching the pipe for a torque pipe. All IMO, of course.
That said, I've never figured any KDX actually needs a flywheel weight. The stock flywheel is hefty as is, and does it's job just fine. If what you're after is more power on the bottom, hold out for the pipe. If you're looking to make it more controllable and eliminate some "snap" up top, then the flywheel weight might help you.