I don't understand the hate in this thread. I can respect a person's opinion that they believe this is a luxury sedan so there's no point in modifying it for more power. But, why even bother clicking on a thread titled "What to do after u go FBO?" To just piss on the OP's decision to mod their car?
OP - No need to apologize. There are a number of people who are modding their 535s because they are super easy to mod and the E90/E92 folks have done all the hard testing already on the N54 motor. So, why not take advantage of their development and see how far a N54 equipped E60 can be pushed?
I drove my friend's SMG E60 M5 and test drove a 6MT E60 M5. The M5 is a great car, but I thought it felt really heavy and the engine was a like a big block Honda VTEC motor (this is not a knock, it is a high compliment as Honda makes some awesome motors). It was fast and it revved like a mofo, but the M5 felt a tad weak on torque. It did not feel as nimble as my 535 or the 550i I test drove. I did love how linear the M5 motor was and how precise throttle position gave you exact degree of acceleration.
With that said, I am enjoying my 535i with Cobb tune. It pulls hard, feels noticeably lighter and nimbler than the M5s and, N54 reliability issues aside, I am sure it will be cheaper to maintain than the M5 not to mention the gas mileage which is massively better.
As for the weight of a 535, it is 3,660 lbs vs. 3560 lbs for my 335is. We are talking about literally just 100 lb difference. Additionally, a 550i weighs 3,946 lbs and an E60 M5 weighs 4,090 lbs. So the whole argument about not tracking a 535 because it's too heavy just doesn't make sense. In fact, a 535i is lighter than a E92 M3 which weighs over 3,700 lbs. I don't think anyone would argue that a M3 is too heavy to track.
Finally, I agree you should get an intercooler if you have not already done so. I plan to do that as my next mod - not necessarily for more power, but to make the engine run cooler and more reliably. Then again, you are running meth and it is arguable whether an intercooler would give you any benefits. If you are running meth all the time, with the power levels you are making FBO, I would skip the intercooler and step up to a LSD. That will allow you to put all that power to the pavement.