The athletic discipline of weightlifting requires a lot of technique. Wether a character has an appropiate active skill or (my favorite) gains additional dice for the weightlifting test from a preceding knowledge skill test. Additionally there is quite some difference in the ergonomics of a car and a barbell. I guess the latter one should allow some bonus dice for being better balanced and providing a better grip. I guess for an unaugmented world-class lifter in SR dice pools of 18+ sounds realistic to me. Nevertheless the game mechanics on p. 152 of the SR5 Core Book are a bit conservative, especially because the game is meant to be quite cinematic.
The formula for Carrying works for me as field kits range from 20kg - 60kg. Lift and Lift Overhead both could be increased by 5kg. Based on STR 6 and dice pools of 18 calculations would be:
= LIFT ================
(STR x 20kg + 20kg/hit)
-----------------------
Min. 120 + 20 = 140kg
Avg. 120 + 120 = 240kg
Max. 120 + 360 = 480kg
= LIFT OVERHEAD =======
(STR x 10kg + 10kg/hit)
-----------------------
Min. 60 + 10 = 70kg
Avg. 60 + 60 = 120kg
Max. 60 + 180 = 240kg