Restricted???

[Silverlegacy]

Air filter would be the first port of call next.
Order a K&N, it's a worthy upgrade regardless if it's the cause anyway.

I will have a little look around see what sort of prices there going for, not had much experience buying air filters but K&N seems to be the name that keeps cropping up.

Count the number of teeth on each sprocket, I know you say they have been replaced? But maybe if they were wrong in the first place maybe they got changed like for like

Will check this in a short while, although i think it was only the rear sprocket that was changed when the chain was done, i took off the cover to the front a while back while cleaning bike and it looked in pretty good condition but didnt look too new.