8 speed flat bar set up with Dura Ace RD-7700 or new Claris RD-2400
The 7700 is higher quality when new than a new Claris derailleur . In particular jockey wheels are sealed bearing and more durable than those on cheap r/ds sold today.
http://www.disraeligears.co.uk/Site/Shimano_Dura-Ace_derailleur_7700_GS.html
A new rd might be better than a worn out old one with worn out jockey wheels (which you can replace), but inherently the 7700 is certainly better, and there probably isn't a better (when new) rd today that will work with your shifters
You can replace shift cables and outers and this will often give improvements without changing any other parts, depending on when they were last changed.