Wow this thread is awsome!
I have to say that ALL of the heads I have ever seen (just got done swapping heads for another member), the exhaust valves had the sandy erroded look as described in the signs of detonation section. I even see this on valves ot of stock engines. I had to polish the valves to make for a smooth enough sureface for my suction cups to hold onto so I could lap the valves. The intake valves had a lot carbon deposits on them. This says to me that the temperatures across the head are NOT the same. It looks like the intake valves aren't getting hot enough to burn off the deposits increasing the potential for hot spots (from leaky injectors/running rich/oil in the intake charge from leaking turbo seals or the PVC system) and the exhaust valves are getting too hot causing detonation.
I see several problems here. The main ones being a lack of maintenance and tuning capibility. With the intake valves, make sure your injectors aren't leaking and have excellent spray pattterns. In my conversion with a representitive from RC Engineering, they recommend cleaning the injectors every 80K. I'd like to see this service be added to the 60K service personally. All ECUs have the capibility to adjust injector duty cycle. But only to a certain degree. In my auto classes I was taught that amotn of adjustment is about +/- 14%. So if increase the injector size (OEM size is 360cc/min) past 410cc/min injectors (360 plus 14 percent equals 410.4), then you just maxed out your ECUs ability to compensate the ijector duty cycle toward the negitive side. Your engine may still run desent, but you will start to get lower idle quality and a driveabilty issues. Some sort of fuel controller will be nessisary at this point, but even a fuel controller on a stock 3/S can give a worth while power increase if you know how to tune it properly. (Spend a lot of time in the ECU tuning forum, on the dyno, and looking at gauges.) EGR can also introduce carbon build up to the manifold and combustion chamber bt
Keeping oil out of the engine seems a bit harder. Valve stem seals come to mind. It isn't always in the budget to replace them but until you do you may as well as save the mod money for maintenance money instead. I have seen several cars with various mods run like crap at the track because of leaking valve stem seals. I don't know what exactly it does to the combustion process (not just octain rating but also causing abnormal/incomplete combustion), but I know it sure doesn't help power production. Properly maintaining your 3/S WILL prevent power losses and make it easier to tune as well as run smoother.
The other source of oil that comes to mind is through the PCV system. A restricted PCV valve will cause an engine to injest oil through the rings and/or valves stem seals. Cleaning or replacing the PCV valve can help prevent this. You can still get a certain amount of oil throgh the normal operation of the PCV cyctem though. An oil catch can can help. It is important to remember that the catch can has to have enough air flow capasity through it or it will act as a restriction to the PCV system. I helped one member diagnosis a hugh crank case pressure proplem not too long ago that we traced back to the addition of an oil catch can. It did not provide enough air flow capasity for the PCV system and resulted in crank case pressureization. The dip stick was shooting out, the front main was pooring out oil, and the engine was spitting out light blue smoke. The low in power as a result of this situation was so bad it was practically worthless to even try and accelerate.
I'm not sure what we can do about the exhaust valves sort of swapping them out for ones that run at a cooler operating temp or transfer heat to the head faster. I suppose if we keep them from getting so hot in the first place (ie keep the EGTs down), then that may help. One way or another the failures and signs I personally am seeing and hearing about suggest detonation and not pre-ignition is what we should be mainly concerned with and trying to eliminate.
That being said, plugs are more likely to cause pre-ignition and not detonation. At least directly anyway. True the head and cylinder walls are the main means by which heat is transfered into the coolant but the plugs still make a significant difference, even if they are not THE MAIN difference. Still, I have seen more evidence of detonation around the exhaust valves than the plugs themselves.