Pis tu fais quoi finalement?
Did you bought a frameset or the bike that you are doing some upgrades?
Blindfold, nobodies can see the difference between 105-ultegra. I doubt people can see a difference between Tiagra-Ultegra. The only place there could be a difference is for the crank and front derailleur, but not for weight but front shifting performances. There could be some difference in shifter precision too (the distance you have to move the shifter to actually shift).
But again, if spending more makes you ride your bike more; you’ll end up faster!