Absolutely! The turnbuckle should match the strength of the rigging wire. 1/8" SS wire has a breaking strength of 2200 lbs and 5/32" wire is rated at 3300 lbs. How much does your boat weigh? When the boat is heeled way over in a blow the strain on the shrouds could conceivably be as much as the boat weighs! Of course this is distributed between the upper and lower shrouds, and there are other factors involved. But clearly 400 lbs is not nearly strong enough.
I am presupposing your boat is a small trailerable cruiser weighing 2000 - 4000 lbs. If it is a daysailer weighing much less you might be able to get by with the lower strength turnbuckle. Much will depend on how large the sail area is.
Also, what are the consequences if the turnbuckle breaks and the mast comes down? Would it be just an inconvenience or would it be a major disaster? Could anyone get hurt?