Studboy is right. You're talking about bikes with 40-50 lbs of difference. I had my DRZ stripped and as light as I could get it without going full Titanium, and it was still HEAVY. My 426 feels like a lightweight compared to the DRZ, and my 426 is still 15 lbs. heavier than a 250 2-stroke!
About all you can do is go with aftermarket exhaust, strip the lights, and yank the charcoal canister. That might shave 8-10 lbs. off the bike. Remember the DRZE/KLX weighs 262 lbs. dry. A YZ426 weighs 231 dry, CRF450 weighs 225, and 250 2 stroke weighs roughly 215 lbs. The DRZ/KLX 400 is heavy.