Coatings, bolts and torque settings

How do various coatings effect bolts and their respective torque ratings? As an example, I'd like to get the external engine bolts recoated. There are a few options for coatings some touting low friction properties and I'm wondering how they may if at all effect the factory torque settings?

I've never used coatings on bolts myself so it's hard to offer too much advice unfortunately. I'd be wary of a coating that offers a reduction in friction as ultimately this could affect the clamping load applied if the existing torque settings are used. This is more relevant though for mission critical fasteners that would normally reside on the inside of your engine. Most of the external bolts are much less critical and are there solely for the purpose of holding parts to the block. An exception here would be the likes of a Subaru engine where the case bolts are accessed externally. If you're just looking at a cosmetic coating for non-critical bolts then I'd say go right ahead.