the cats we have are either weld in for pretty much any car or we have a bolt in cat to suit FG to suit standard dump. Tyres as well as wheel size can make a difference on the dyno. I have expressed this to the OP, but the car was presented to be tuned with a combination done by the owner, I know it has gone back for other issues, but it is not the tuners job to sort out why it's not making the power when they did not do the combo assuming the tune has been done correctly. If the tuner had supplied a package to make XXX amount of power then it is their job to make sure it does it and if it doesn't then they need to find out why, but if a car is presented with a combo done elsewhere it is just their job to tune it, if it doesn't perform it's not really their issue, but they should be willing to chase it but this may come at added cost.