Usually the loss of power is just the lean burn requiring more throttle because you cut back on the fuel when leaning the A/F mix and to maintain the needed power levels you have to add more air to get the fuel flowing again at the leaner ratio and you should net higher MPG as an end result. If you don't have a NET MPG increase then you are running too lean and not saving fuel - probably because the fuel is not burning completely.
The lean burn should not hurt the valves too much as long as you are not pushing high horse power levels since they are designed to run hot. Just watch out for high head temperatures or over heating. Water injection can cool things a little since the valves get cooled by the incoming air mix. Also lower RPMs will reduce the exhaust valve temps by allowing the fuel to burn in the cylinder before the exhaust valve opens where as at high RPMs the fuel is dumped out still burning past the quicker opening exhaust valve.
Good news, JanGeo.
I'm going to get a temp gauge, and take a baseline of my head[s], and exhaust (< if the gauge can do it), after running at 65 for a while to get a baseline, and then take them again after leaning it, and advancing the timing.
What would you all suggest the temp change limit should be?
The engine was designed [by Porsche] for performance, and redline is at 7K rpm, so I assume it's built to get hot. I've actually never heard of someone burning a valve on the forums specific to the engine (KLDE, KLZE). I'll ask specifically, though. There's a drag racer that races his at around 10,000 rpm almost weekly, and has had it up to 12,000. I imagine that gets hot!
And, the cool thing is that I ONLY lean it at idle (15.5:1), and cruising. If it does get hot, I can just slow down, where the AFR is about 15:1 (54mph).