First of all, nice avatar!

As far as Dinan specifically, quite a few guys on here have "Dinan"ed their E53 V8s, myself included. I think you will find that almost everyone has been very pleased with both the performance and the looks. You can do it piece-meal, or all in one fell swoop. Basically we are talking about 1) cold air intake, 2) throttle body, 3) exhaust and 4) Stage 2 or Stage 3 software depending on which bolt-ons you have installed. You can also add a Dinan strut bar as the icing on the cake. Please leave all illusions of beating Cayenne Turbos or Jeep STR8s back in your dreams. They will all still smoke you out of the toll booth. BUT: You CAN expect a modest 25-35hp boost with the above combination. Dinan thoroughly tests and warranties all their mods, so whatever horsepower numbers they post tend to be the real deal. And you will definitely feel a difference in terms of throttle response, sound, and acceleration. Dinan is certainly not the only or the cheapest way to go. You can save money by doing a-la-carte from different mod-makers. For example AFE intake + Supersprint exhaust. I just found Dinan convenient because it was one stop shopping and installation. But it ain't cheap!

One other thing to keep in mind. I did my Dinan mods 3 years ago when there were no X5 twin-turbo V8s on the market. At that time I did not regret the cash outlay because the E53 4.4i was still considered a mean and fast BMW SUV. But times they are a-changin'... If I had to do it all over today, I would not invest that much into an aging E53 chassis. I would simply save the Dinan money towards a down payment on an X5 50i. But that's obviously dependent on your budget and how long you expect to stay in your 4.6is.

Hope that helps!