my take on groups and levels.

shifters
usually the absolute best shifters have more crisp and exact feel even compared to the number 2, big difference.
then it gets progressively more mushy and indistinct the lower you go.

cassettes
the more expensive ones seems to last longer.
but... the 2x more expensive one will not last twice as long, maybe 50% longer

cranks
the best ones save a few grams, but when all the chainrings are removed were talking 50-100g, go figure

bb bearings
yesterday I tore apart one of the most expensive hollowtec 2 and also the cheapest one to lube up correctly with molykote. it used the exact same bearing, looked exactly ythe same inside, had the exact same rubber seal and and the exact same plastic T-bushing. Only difference was the color.

derailleurs
the most expensive stuff weighs less, but they do the exact same job, my tiagra rd with steel cage is the most prositive shifting one I've tried. its like clockwork.

brakes
the best ones save weight and functions very good, the cheaper ones work just as well but is heavier, and the cheapst ones work less well and is heavy. durability can be concers with the cheapset ones too. get something from the middle of the foodchain

chains
get the best ones from kmc

chainrings
get steel

i get these parts:
cheapest cassettes
second or third cheapest derailleurs
best chains
cheapest steel chainrings
middle of the range brakes
best shifters/levers
cheapest bb cups filled with good grease
best cranks (for the bling)