I'm no expert on them, but I do own a 2005 V8 mustang GT. You mention 2004 onwards having a quad cam 32v 4.6 modular engine, that's not quite right. As far as I know, the 4.6 V8 is a single cam 3v per cylinder unit with VVT.
As for tuning, yes, you can turbo them, but the big results seem to come from supercharging them. From quick chats with fellow owners, around the 550-600hp point is the limit for the stock internals. The 5 speed auto is at it's threshold just under the 500bhp mark, whereas the manual box fitted to the GT is considerably stronger.
The thing with these engines (witnesses from supra pod can vouch for this ) is the low down torque figure with no lag means they leap off the line, even when bolted to a fat convertible S197. Their top end is no where near the ferociousness of a tuned 2jz, but the fast pick up more than makes up for it.
To put it in perspective a bit, my stock mustang runs around the 14.5 mark on the quarter mile. An identical car with bolted on twin scroll charger runs 11.9.