[Ollie]

I've just hand cleaned until now- still a bit wary. On the performance side, it probably is better for the engine to have less gunk on the outside. It might not be as important as on an air-cooled motor where the cylinder fins, etc. should be kept clean to allow heat to dissipate quicker and so on, but eliminating heat-retaining crud is probably good medicine for any engine.
For things to tight to wrap, I wonder if electronics cleaner would be a good pre- or post- barrier/wash?