A proper island hood should be 36" wide over a 30" cooktop. A downdraft exhaust should only be used as a last resort. Furthermore, it will pull on the gas flame of the burners. But most of all, you will likely void the Wolf warranty if you have a downdraft with a Wolf range. If you go to the Wolf website, you will see under the FAQ section:

"Can a downdraft be installed with a gas range, gas rangetop or dual fuel range?
No, the downdraft is not compatible. Wolf recommends a professional series hood be used with dual fuel ranges, gas ranges and rangetops. A downdraft can be installed with a sealed burner rangetop (Models SRT304 and SRT366.)"