The shift changes should be quick and hard in 1st, 2nd, 3rd and 4th on WOT.
A sloppy gear change with a hot gearbox after the shift kit is installed and under full throttle especially when the car was shifting OK before can mean 1 of 3 things.
The wrong oil has been used when refilling......your gearbox will die pretty quickly so confirm exactly what oil has been used with the shop.
Incorrect oil level in the gearbox (check he put in about 7.3 to 7.5lts).........get the gearbox warm, put it in park and check the dipstick.
The shop stuffed up when installing........your screwed as they will never admit it!
FYI - out of the all the re5 Transgo Kits I have purchased and installed(40+) I have never had a part missing.....which leads me to believe the shop has stuffed up!