lol i worked at a tire shop for a long time we put 205 50 15 on bagged trucks many times it was fine. ive never heard of anyone blowing the tranny out with smaller tires unless they are power braking it everywhere, they have high mileage, or tranny fluid was never changed. as for just the tranny going out when you put low pros on nah i dont think so. best thing to do is just smell or change the tranny fluid every 30000 on the 4l60es. the only thing that will be off is your speedo.
i have 275 35 18 which is 1.5 smaller than stock tire sizes, i was running 11 and 12s with my old motor tranny never got messed up, street raced on weeked on slicks that were about the same size tranny never blew up, blew the motor up put a stock one in on a 75 shot ran 13.3 on slicks, tranny never blew up, street race now on 18s and i abuse it like hell tranny still hasnt blown up. the only thing the lower profile tires will do is make your rpms go up about 2-300 on freeway with your set up. i had 410s and lost 600 rpms on freeway and my tranny was still fine, many others also went with 373 or 410 on this board the tranny is fine.
lol gettin the tranny recalibrated for smaller tires only has to do with the sending unit NOTHING INTERNAL. when people have speedo recalibrated with Hypergtech programmer it just messes with the speedo electronic sending unit, nothing inside changes