I want to improve the shi*ty gas mileage that come with my 4.4L V8, so I knew I had to upgrade some stuff that had to do with performance. The following are the things i WANT to upgrade, but I haven't seen yet: exhuast SYSTEM, intake (not just the filter), ECU, and MAYBE, a super/turbo charger. If anyone know where I can get ANY of the parts listed above for my 97 bmw 740i, i hellaaaaa appericiate it.

To be frank with you,i suggest u just install a 2.5L engines with transmission, if you wanna safe fuel.
The weight of the car is about 1970kgs.2.5L can drive u around safely and economically.Pleasure of driving will be there definately.
Some guys have fitted nissan cefiro engines.Am against it.
Changing and modifying 740i is not going to make u happy,simple reason is that u have 4.4L and only arab sheiks can afford this days coz of fuel cost.
If u think is not prestiage issue,then sell it at good price and buy the smaller ones below 2.5L.
Choice is entirely yours.
Sorry if I sounded rude.

Dinan doesn't carry your performance upgrades anymore, however Bavarian Motorsports still has parts to enhance performance (link below).