I tried doing flipped manifolds on my TT440 back in 2001. They blocked the spark plugs when installed upside down and interfered with the engine mounts when swapped side to side. If it hadn't been for those issues I would have used the irons.

In a typical street turbo car the manifolds should do fine, assuming they're in good shape.

They will be really heavy but if there are no clearance issues go ahead and do it. The iron will actually keep heat in the exhaust and work the turbos a bit better than thin wall headers.

Pretty turbo headers make a bit more power but it's not an earth shattering difference in most rides.

If you do it, go EFI. You'll be much happier on the street.