It has never worked out for me, but I maybe wasn’t so canny. Mrs. Squirrel has a Cube full suss with RP23, full XT, Reba U-Turns and awesome finishing kit (such as a £70 saddle) – £1200. We wouldn’t (and haven’t) changed anything. We got to £1200 adding it up really quickly and still hadn’t accounted for most of the bike.
Depends on the brand but with some bikes, the manufacturer is cutting corners and you might need a new headset, seatclamp, saddle, brakes etc. quicker than you think thereby resulting in a false economy.
I’ve come to no conclusion.