I did the ceramic coating to one of my turbos (I have several) but I dont notice any difference, not that I would.  It flakes and chips off over time.  So I mostly just rely on the stock turbo heat shields.  They do eventually wear out, but I bought several cheap used sets on e-bay over the years so I have a ready supply.  They are made of some sort of steel alloy with a steel mesh liner as insulation and seem to do the trick much better than the coating.  So yea, a "bonnet" is what I recommend.  Anything that gets close to the turbo gets a piece of SS (or even aluminum in some cases) to act as additional shielding.