The top of the mast would have to be in the water for there to be a threat of the boat capsizing. A lot depends on wind and wave conditions but the boat would have to be on its side first.
As to maximum wind, like any sailboat it is a matter of reducing sail area as windspeed increases. Theoretically, a C-30 like any other 30 footer, could survive hurricane force winds properly sailed under bare poles (no sail up) or a small storm jib/storm trysail. The key words are "properly sailed".
Are you planning to sail into gale or hurricane force conditions anytime soon? Generally, you will want to start reducing sail area by reefing or going to smaller sails when you start to feel uncomfortable or the boat is heeling over excessively and you're fighting the helm to keep her moving in a straight line. Reducing sail will reduce weather helm and make the boat easier to handle as wind speed rises.
Although I don't own a C-30, you should be safe under 25 knots of wind provided you feel comfortable sailing in those conditions and have reefed your sails appropriately. I doubt you'd be safe sailing into that wind with the main fully up and a large overlapping 150% genoa. The boat will be way overpowered, be placing excessive strain on the rigging and you'll be fighting her. With one or two reefs in the main and the jib reduced, you would be able to sail.
SV South Paw
2000 Hunter 340
South River, MD