Paid setups generally offer a significant pace advantage over default setups mainly because they've been adjusted by expert drivers/engineers to optimise the settings for a specific track. The problem with this is that these setups don't come cheap.
Does it make such a big difference?
We wrote previously about paid setups in Assetto Corsa Competizione and during our testing found a big step up in pace between the default Aggressive setup and paid setups from different setup shops. Our experience has shown that with ACC, setups from setups shops are a lot faster with some shops producing better setups than others.