Diamonds don't like too high temperatures or humidity. I would say 30 is too hot for diamonds. Might not see any short term effects but in the long term it is questionable whether incorrect setups are causing of various problems diamonds seem to have in their older years. Many seem to deteriorate from 8 years or therabouts, for no current, obvious reasons. A good temperature gradient for a diamond would be 26-27c hot end, 18c at cool end. If you aren't getting the cool end cool enough (say if its always above 20) then you might need to upgrade to a slighty bigger click clack.
I know many have argued that where diamonds roam wild temperatures do and have gone over 30c. But remember wild diamonds are free to find themselves a cool spot that will be no where near the same temp as is in the sun. Our diamonds are trapped in enclosures therefore it is up to us to make sure we lookout for their needs as best we can.