I agree with Simple Green. Rinse with a water. However, be careful not to get any in spark plug holes, on alternator, or any other electrical connections. Do it when cold and then drive it afterwards to dry it good.
I do it primarily for looks but definitely helps in seeing leaks, etc.
I dont see much performance aid other than making it more pleasant to work on the engine - so maybe you will try that CAI or whatever.
I bag the alternator, distributor, fuse boxes and exposed wiring harness connectors, intake filter if there and tape the bags down well.
Spray with a good degreaser when the engine is just warm
let sit for a while and hose off with a gentle stream - you dont want high pressure
get a toothbrush or a rag and clean out the grime on the gasket seals if any
reach under and get all the hoses clean
spray with degreaser again
wait and rinse let sit
remove the bags and inspect for water
when dry inspect all your hoses and treat with silicone lube - it restores them and they go black and the engine bay looks hot.
see photos of clean engine - search for intake filter done and follow links to photo page - there are some spanking clean shots.
As for me, I just bag my distributor cap and air filter. Spray it down w/ Simple Green. Let it set in for like 2-3 min, then hit it w/ the pressure washer. After that, I come by with my air nossle and spray dry the engine with shop air. I hit the battery first (so corrosion is start up bad) and then hit the engine.
I guess I'm a bit overzealous under the bonnet but the last time I washed it the water must have got to my temp guage sender unit connector - it corroded so badly I had to replace it - and there's not a lot of slack cable in the wiring harness!
Better safe than sorry.
btw had every panel keyed on what was an almost immaculate G1 a few weeks ago.
Very, very pissed off.