I've only recently had some hiflo-catted ARD downpipes fitted and love them. There is a slight smell but not noticeable inside. Noise wise mine sounds almost stock..till you give the right pedal a good squeeze (at motorway speeds its as quite as stock)
. I went for the catted version purely for emissions/MOT reasons but plenty of people have the full decats without any problems at MOT time (the full decats also get more power).
Quaife LSD is fab if you got the cash spare, since your in London Birds BMW will get the it fitted without problems. The diff allows you to put the power down much more easily and stops the annoying TC from coming on every time you touch the throttle. Whilst your at it get some M3 subframe bushes and better antiroll bars fitted at the same time since the rear subframe has to be dropped for the diff (my car is with the garage now having the latter two fitted...should have got it all done at the same time as the diff but didnt have access to the funds till now). Google "Rear Antiroll bar 335i" and your see just how pathetic the stock bar is compared to aftermarket versions...
If your now going down the non-BMW route from what i can understand the BMW Performance brakes aren't much better than the stock 335i brakes (they do look nice though). AP racing do a nice big brake kit but thats very expensive, alternative is to get better pads/brakelines/fluids which should cost less than the BMW stuff and offer better performance (again my car is in the garage getting this lot fitted).
The JB4 is a great bit of kit, I waited for my AUC warranty to run out before fitting it to my car and have to say should have done it sooner. The extra BHP is quite addictive!!
Be warned theres lots and lots performance bits for the 335 (most of the good stuff isn't from BWM though) ...set your self a budget otherwise you soon finding your self spending lots and lots of £££, I've already gone over my budget but only by a few ££ so after the car come back from the garage thats it.....though Koni FDS shocks do look nice