I think he means that you remember how good they are on your mountain bike compared to the cantis on the cross bike.
Those spesh slim shims look just the same as the standadrd shimano spacers that come with the levers/bike. If they didn’t come with the bike, ask the shop to give you them.
I agree about set-up, bar shape etc.
What takes a bit of getting used to is descending on the drops. It’s a bit scary at first, but it puts your hands in a much better position for braking than trying to descend on the hoods, which initially feels better because you’re more upright.