Hi mate i'm new to the site but I have some knowledge on the 2.8 engine tuning possibilities as my brother has a modded 528i, the induction kit i would recommend for you is a Gruppe M kit with a carbon fibre heat shield, these can be bought from CA Automotive who specialise in bmw tuning, this would give you between 7-10 bhp and more mid-range torque.
another mod which I would recommend is a set of power pulleys, these are relatively cheap at about £125.00 but give an extra 10bhp and improve the throttle response at lower revs. Apart from these mods you could get a stainless steel exhaust and an ecu re-map. If you really want to break the bank you could get a turbo or supercharger kit, a supercharger would cost around £6000 and an extra £600 to fit, this would take the car from a standard 193bhp to 301bhp... not really worth it i thinks!!!