sorry for the late response. i didn't get notification of any activity and decided to check in this morning.
there are a lot of things going on in these drivelines and to make matters worse, there are people controlling them. without the car in my shop i can only take stabs at it.
bad synchros will generally cause audible complaints. in most cases getting in and out of gear is absolutely possible and noisy. i have had rare occasions where synchros get partially engaged and actually mushroom over the teeth and close up the gap enough that you cannot get into gear no matter what. it just grinds. sychros go bad as a result of missed shifts and bad clutches...and age, but nobody can seem to get enough age out of these things to see a synchro fail naturally.
if it goes into gear, but grinds at higher rpm or under fast shifting and not low rpm and slow shifting, the synchro cones are shot. the cones are what attempts to match rpms between the snychro ring and synchro so they can engage. if it takes less rpm or slower shifting, the cones are worn and unable to get the rpms matched up in the time given. these often times will show their wear in inspection, but i had an instance where my 5th gear ground no matter what. rev auto took the trans apart and ronnie and i both inspected the 5th gear synchro, cone, fork and fork pads and saw no wear whatsoever. it went back together carefully and did the same thing. brought it back and replaced the cone that looked perfect and 5th gear was functional again. its weird, but the synchros and supporting parts are so incredibly sensitive in these MT82's.
lockouts are another issue all together. we need to be clear about the terminology here. "lockout", as i define it, is when you are trying to get into gear and it will not go into the gate. it doesn't make noise. then, after the rev's settle down, it goes into that gear just fine.
assuming that your definition matches mine, lets talk about lockout. why it happens and how to correct it. first, the shifter geometry makes the gate spacing narrower than a Tremec. single fulcrum short throw shifters make it worse. the other part of this is lead in to the gate. its the gate's funnel of sorts. this is controlled inside the transmission. i have seen first hand, the vast difference in lead in on the MT82 verses the TR6060 and its drastic. furthermore, the TR6060 has a detent ball path that the gates and throws operate in, so theres sort of a mechanical guide for the shifter to follow. the MT82 just has the shift rail tabs and slots so it requires more feel to properly engage. all this means that the driver must be more precise in placing shifts. if you can be precise, you have a better shot of high rpm and fast shifting, but until it becomes repeatable as well, there will always be missed shifts.
i have the ability to repeatably powershift my MT82 at 7800-8000 (my youtube video from earlier this year proves it). I have no special clutch (spec stage 1), no special shifter (stock), no special synchros (none available), no special internals (just a brass shift rail stop pin and factory plastic fork pads). i can do this because i have spent a lot of time becoming precise in my shifting and worked hard to be repeatable. i too, have occasaional lockout and grind synchros to dust from sloppy clutch pedal work. i know it was my fault when that happens. it gets exaggerated when on track because i have so much more to focus on than just my shifting. so, i slowed my shifting down on track because i am not capable of managing it all with total accuracy.
here's a story: customer calls with shifting issues even after buying an MGW shifter (non-race spec) and hopes the shifter support will cure it. i tell him it will. he buys it and installs it. he calls me and tells me it didn't work. he was fairly local, so i told him to come by and let me evaluate the situation. he brings it by and he tells me that when he goes to the drag strip, he is trying to "bang gears" and gets constant lockout and can't manage to get better than a 13.50. we go for a drive and evaluate. i take it easy to analyse and can't find anything binding, so i push it further and further until i was poswershifting his car with his shifter at 6800 (gt manifold) first through fourth. he was astounded. he had a long life as a manual driving drag racer, stepped away from it for several decades and bought this mustang to give himself something more to live for after his wife of 38 years died. he vowed to relearn how to shift this transmission and the last i heard, he slowed things down and clicked off a 12.80. i hope he has continued to improve as time passes.
in that case, the car had nothing to do with the problem. that doesn't mean the next time that happens, the car won't be. its just a lightbulb moment to hopefully give everyone cause to pause and evaluate if it really is the parts we throw our hard earned money at that fail us or its ourselves that fail us.