I'm buying a new drivetrain (cranks, cassette, chain) for my flatbar commuter that was mostly built of parts. I currently have a Dura Ace 7700 short cage derailleur on it, I got it from a friend a few years ago after he rebuilt it. I've been using it with Claris 2x8 index shifters, and the shifting has been so-so. I'm swapping out my old Ultegra/600 FD for the fd-2400, because I know there are issues with flat bar shifters, road chain rings, and [old] derailleurs. I'm basically wondering whether I will have better performance from a new 8-speed derailleur (because of trickle down tech over the course of 17 years), or from my old Dura Ace 7700. I mostly use this bike for fast 12 mile commutes in NYC and weekend overnight (much slower) rides with my wife in the surrounding area.
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.
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.
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.