2 liter bottom end 

?
How much would I be looking at for that? What sort of performance gain can I expect?
I think you get more torque than HP out of that.
Also a different cam, I'm not sure which is best and it seems some people have different opionions.
A good service will also see you good
As will a decent air filter, they give you maybe 2 or 3hp, but will apparently make the throttle repsonse better.
Drilled airbox also makes the throttle feel better.
A higher octane fuel will also ever so slightly increase power, you can get fuel additives to increase octane but if you're using 98/99ron already I'm not sure how that will effect your engine. Try running Redex through the old gal to clean her inards, I usually run a shot once a month.