The Cat should be the last resort in solving a high HC Level.
If your mechanic tested everything else, he or she is probably right.
More likely culprits are components that affect:
The Cat is only there to deal with residual gases from incomplete combustion. Reduce these residual gases, and the Cat has less 'work' to do.
Make sure your car is completely warmed up before testing to give your Cat it's best chance of keeping up.
With some work, the exhaust going into the Cat could be compared to the exhaust coming out to show whether or not it is providing an adequate reduction in HC.
This is a good link on Cats: