They're rubbish, I've ridden a Hornet 250, it reved past 18000 (not a typo, that's eighteen thousand revs) but wouldn't pull the skin off a rice pudding, a std '28 woud be quicker!

CBR250 claimed specs, dry weight 158, a good 30kg more than an NSR, I'll bet 90% of that is engine.

It's not going to weigh less, you can carry and NSR engine yourself easily, not so with a CBR250 or 400 engine and box.
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