Other end of the country to Bremen. 505 has a full service history, due an oil change now. I can't imagine a 4.6 would be down on power after 45.000 miles. You do have to press the pedal down true the carpet to get max power but it's still lethargic. Maybe I'm just spoilt. My pickup has a 5.4 with 300hp and the Jag has a 4.0 supercharged with 370hp.
I'm looking into better flowing heads. Most of the massive power increases we've been seeing over the last 10-20 years has been down to better understanding of flow and the resulting better heads. If I find something I'll let you all know.
Ed