The short answer is yes, re-capping vintage speakers can dramatically improve the sound depending of course on the resolution of the speakers, and how degraded the caps are. Here's why. Generally once a loudspeaker is 20+ yrs old you would expect the caps to start degrading. Internal wire may also start to oxidize. It's also worth getting the bass drivers serviced, if only to get the voice coils re-aligned. If the boots are made of foam (in lieu of butyl rubber), they would be checked for drying and cracking & replaced as required. Then once you get to 30+ years old, solder joints can start cracking. For example, the old Infinity IRS and Reference series speakers from the 80's have an issue now with delaminating magnets and dry/cracked solder joints.

The above issues can send drivers out of spec, impact resolution and bass response, and generally muddy the sound. Also, re-capping 20-30+ y.o speakers allows you to use much nicer caps/inductors of the same value such as Mundorf, Duelund etc which have much greater resolution and musicality.

If summary, If I was considering re-capping the x-overs on a 20-30+ yo pair of speakers I would do the following things;

- Replace caps & check the inductors (ideally replacing as well to match the higher end caps performance)
- Re-wire the x-overs with fresh connecting wire
- Having the bass drivers checked & v/c's re-aligned. Replace the boots if necessary.
- Replace internal wiring with some nice wire such as Cardas or Furutech
- Replace or clean binding posts with 99.8% audio-grade isopropyl alcohol or Deoxit (If oxidization or tarnish visible)

That said, you'd have to consider how long you plan to keep the speakers & if you think it's worth the expenditure. But my take is, if you're already opening up the speakers to refurb the x-overs, you might as well re-furb the whole speaker. Hope that helps!