Development from Playing with Others
While it's probably hard to make qualitative statements about how much faster one develops when playing with a group, I do think we can narrow in on what that development looks like. In particular, I think there are some specific musical skills that develop quickly when playing with other musicians, while other skills are best developed alone.
In a jazz context, the skills acquired by playing with others are centered around listening to your bandmates and responding to them. For example, when improvising, the soloist must respond to the rhythm section's dynamics, rhythmic complexity, harmonic variations, etc. If you are a member of the rhythm section (piano, guitar, bass, or drums), you'll learn to follow cues from the soloist and your fellow rhythm section members. For example, if the saxophonist is soloing and suddenly plays a loud, high note, you need to know whether to respond and how to respond to that. If a horn is playing the melody and is sustaining a single long note, you need to know how to add a fill--or if someone else starts adding a fill before you, you need to be able to listen, hear that, and lay back. As another common example, let's say a horn is soloing, and the second time through the form, the bassist changes from playing half notes to walking a bass line. The piano/guitar usually wants to complement that shift by adjusting their comping rhythms and comping style. If a soloist starts playing a polyrhythm in their solo, the rhythm section needs to know how (and whether or not) to respond.
There are all sorts of on-the-spot maneuvers like these, which help provide cohesion to improvisational music and which provide good contour to a solo or song. These skills contrast with things that are best practiced individually, away from one's bandmates. You haven't asked about this, so as just one example, if you play jazz piano/guitar, learning voicings is probably best done alone (when not on the spot, musically).
To your question, how much faster does musical development occur when playing with a group, I think the answer is that certain skills don't improve any faster, while other skills improve infinitely faster because they can't be practiced outside of a group context. For instance, even if I put on headphones and played along with a famous recording, the musicians in the recording obviously can't respond to what I play. So I can't think of a way to simulate the interactive environment that a real group provides. Without a group to play with, certain skills won't really develop at all, while other skills will continue developing at mostly the same rate they would if you were playing in a group.
As a caveat: upon reaching a certain level of musicianship, I think one gains greater ability to practice individual skills in a group setting. When I was in college, I saw Ingrid Jensen play in a very small performance setting. I had a conversation afterward with our jazz director about how she was practicing certain improvisation techniques throughout the show. She had certain motifs she kept coming back to, and she was working out how to use and develop those ideas in various contexts. She probably doesn't do that at every concert, but it was interesting to see her working on a specific improvisational idea during a concert. (And it didn't take away from the performance at all! It led to some neat insight into how she practices motifs.)
Quality of Musicians
In improvisational music, these techniques form a sort of vocabulary for playing as a group, just as licks help form a vocabulary for soloing individually. As you play with better and better musicians, you'll be exposed to more and more techniques. But as with anything, the place to begin is with the fundamentals. So you're absolutely right that playing with more advanced musicians is more beneficial, both because (i) you're exposed to a wider variety of techniques and (ii) the bar/accountability for performing the fundamental techniques is much higher. That said, any chance to practice something new at any level can be beneficial.
Finding People to Play With
A local college/university is a great place to find musicians! At my school, we had a small-to-medium-sized jazz program, and it wasn't too uncommon for us to draw on local musicians when we didn't have enough rhythm section players for the combos. You could reach out to the jazz director and offer to play in a group if there's ever a deficit of your instrument. It might be hard to break in, though, without some connection to the school. So here are some things you might try to help warm up that connection:
- go to the school's concerts and become a regular face around the program
- befriend the musicians in the program and show up to informal jam sessions
- take lessons through the university (you might be able to ask in advance: "if I take lessons, can I play in a small jazz combo?)
- take a music class at the university (this might be a sure-fire way to gain access to the music groups)
- offer to volunteer/work music events at the university
Those things might increase your chances of the school making an exception and letting someone who isn't a full-time student join their performance groups.
