raineyuk wrote: it also feels like it drops out of the powerband a bit on each upchange.

What RPM are you changing gear at? With my derestricted (otherwise standard) 21SP, an upchange at 11,000 certainly keeps the engine revs firmly in the powerband.

Also worth checking that your powervalves are properly adjusted. If set-up incorrectly, the power characteristics will be affected. Notes can be found in section http://www.nsr-world.com/tuning/250-tuning/engine/
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30