Thanks everyone for your comments. I'm not looking for RS levels of performance Matt@TYGA, as I don't want to rebuild it every 500 miles - but an extra 5-10hp wouldn't go amiss.....
If I can get 5hp by new pipes / new reeds / airbox mods that would be fine by me; maybe one of the Terry Shepherd NS400R lower pipes (that does away with the ATAC) and a new rear cylinder end can would be a good start?
Unless I can get some 84/85 RS250 pipes and other bits of course....
Next step would be new carbs.....

My understanding (from my old 350LC days) of 2-stroke tuning of bikes of this vintage was that the top end revs were restricted by engine breathing, i.e. airbox / carbs / reeds / pipes. I am sure the NS250 airbox is much less restrictive than the 350LC one because of it's better packaging, so I expect there is no value-add in Ramair filters, and drilled airbox / carbon reeds / pipes would be the easiest gain. Any guess how much I would get by this alone?

I'm not too confident about grinding the ports as I am sure I remember from the time that (on the NS400R at least) that you risked chipping out the Nikosil cylinder coating and wrecking the barrel?

The bike is no slouch as it is (especially on 14 / 44 gearing instead of the standard 15 / 42), but more power is always welcome.........

Thanks too for everyone not shouting me down for it not being a true NSR!!

Phil