Not to unduly step since Todd hasn't chimed in, but it's a false dichotomy between "soloing in 8th notes" and "steady stream of quarter notes".
Rich Perry says something very amusing when he talks to students about improvising; when he's asked what to do when soloing he says "Avoid playing 8th notes for as long as possible". I'm pretty convinced that playing cogently uptempo is more about hearing/conception at uptempo than it is a physical/technical issue. You can play stuff that you've worked out (melodies, exercises etc.) at higher tempo ranges than you can solo at. And there's so much stuff that
if you're hearing it, you go through whatever convoluted awkward **** to get the idea out. You'd do the same thing, fumble the idea out at that tempo. With me, I just don't hear **** that fast.
But as Bird said, when asked about playing uptempo, "I just pretend I'm playing a ballad." Listen for a longer line, bigger note values etc. Playing a solo isn't about regurgitating a million notes, it's about playing something melodically cogent and communicative over/through an harmonic framework. Like I said, I don't hear **** that fast and one of the great things about starting to work with the bow is that 1. I can try to bring out these long singing lines and 2. my right hand (with the bow) doesn't care if my left hand is playing whole notes or 16th notes.
If what's happening is that you have all of these really great ideas but you can't get them out at tempo, then you need to break down what's happening - is it position shifts, is it left hand fingering issues, is it lack of independence in right hand fingering (like do you always have to start an idea with the same finger etc.) are you not really hearing the changes, are you not clearly hearing the idea etc etc. Once you have the "problem areas" isolated, then you hit the shed.
You listen to Prez on some of those JATP recordings and he's playing these really wonderful lines of blistering rhythm changes, but it's not these constant streams of 8th notes....
Oh Todd came back!