8 speed flat bar set up with Dura Ace RD-7700 or new Claris RD-2400

7700 was a 9-speed groupset. at that time, the cable-pull ratio was standardized, such that any 6- to 9-speed shifter, including your Claris, should work perfectly. If your shifting is only "so-so," I might look for alternative explanations. so, as far as performance is concerned, they should both work equally well. it's hard to even say what would come in weighing less, without actually measuring them with a scale.

if it were me, i would probably stick to the 7700 since it's more interesting and you already have it. if you do want to swap it, i might recommend borrowing from the Shimano mountain lineup and picking up an Altus or Acera rear derailer. they feature nice large jockey wheels and they work just as well as any derailer; plus, their longer cages allow a wider gear range. there's a reason you see them on high-end custom frames like Rivendell! nothing wrong with the claris, of course.

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.