Hi, Craig. I actually read the OP about five minutes after waking up and yeah... blurred on the exact nature of the task. Also wasn't taking a swipe or anything... just pointing out to the OP (who may not have known otherwise) that AS transient detection needs to be double checked at the very least.
Thing is though, in regards to the detection on even a guitar track (especially if there are already effects on the track) IME the Markers need to be corrected before bulk stretching is done.
Like I said I just went through this with a current project. The guits were DI and picked at every 8th note (or something like that) conistently throughout almost the entire song. The pick attack transients were quite clear and pronounced and there was no ring out (palm muted chug). The original performance was actually almost spot on with the timeline but the client want it to be robotic (industrial) so I went the audiosnap route to do so.
As expected and often mentioned in tutorials (yours included where I've picked up a lot of my AS knowledge) there were quite a few Markers that were slightly off the actual transients, some were unwanted and doubled up (due to maybe slight string noise) and some were missing (even though the transients were clear and defined).
Just the nature of detection stuff like this and cleaning things up properly before stretching, as you know, is needed beforehand or you get unpredictable results. Nature of the beast.
For groove extraction eq filtering or gating/transient shaping can be used on the source material to make the transients stick out more and THEN the detection works great (and I actually had to do that on this project as well)... but you can't do that on material that is being corrected because then you just ruined the sound and can't get it back after the bouncing.
You of course know all this and perhaps you have some tricks or your equipment somehow gives you better results with this type of thing than I get but I just didn't want a new user to look at that and think it's an totally automatic process. Never is.
However since this is just a backer track that is being used to build around and actually the OP is a little vague about exactly what is being attempted (like is the solo/guitar track the only thing that speeds up and all the other instruments stay in time? Is this a multi track file where all tracks need adjusting or a stereo mixdown? They mention vocals and there is an implication that AS adjustment throw everything else off, etc).
In any case, if it's just a scratch track then doing a clip stretch instead of audiosnapping still seems like the better/easier route to record over top of. I've done it quite a few times before when collaborating on covers. I'm even about to restore one of my old band sessions using these principles (but WAY more in depth and complicated stuff... essentially correct drums and vocals then rerecording everything I can on my own).
Aaaaaand to top that all off audiosnapping a solo (compared to the simple chug I was talking about earlier or a drum beat) is pretty finicky at the best of times for transient detection and correction. It's gonna be a lot of notes flying around in weird ways with different textures and intensities that would need to have the markers combed through to make sure they are all there and in the right place. THEN to do a bulk correction would still be hit or miss unless it's a super simple and rhythmic solo because we all know what happens when you try to snap something like that in bulk. Some notes go one way and others the other way. Things that were in time go out of time. Things that were out of time and end up in time might sound weird. Groove and feel gets lost and it's a TON of work for... well not very good results.
Doing it note by note by hand is the way I would deal with stuff like that but it ain't worth it for a backer track.
Totally not dissing or being argumentative of course. Just putting my real life experiences with this stuff up in the forum archives for those who may search this topic so they can know what to expect and how to solve the issues that may come up.
You know I lurvs ya, Craigo... and really a lot of this knowledge has come from your own tutorials.
Cheers!