Whats the point of sleeving it down to 800?
Weight is still there. Developing a newer engine is very expensive, take a bulletproof one (well, fix the waterpump only one seal issue), remap it, change the cam profile to be smoother and fuel efficient, maybe adapt a multi management mode to be possible to select different types of performance.
Framewise there are already good options, maybe reduce some weight.
Dunno about tax issues, but 800cc locally is taxed exactly as 999cc's.
Growing old is mandatory... growing up is optional!
Raid de L'Amitiť 2011 RR (link