There are not much you can do to the L6 engine that will boost the power. RPI Scoop, KN Filter and Sprint Booster for drive ability.

 

Suggest you skip the Sprint Booster and substitute Burger Tuning BMS Power Box if you're capable of the more complex installation. Same effect on throttle lag, with a little horsepower boost from some mild ECU mapping, not much more expensive. All these are worth the money, but there really is no dramatic engine performance upgrade available for your engine. Other than sell it and buy an M5, which will be someone's response to this thread eventually.

Suspension, on the other hand, you could play around with a little. I'd suggest bigger sway bars and chassis stiffening as initial items worth the money. Front and rear strut tower braces can be found, along with some other chassis reinforcement bars. Cusco and Ultra Racing are two manufacturers. I have done these upgrades to an e60 528, and found them worthwhile.

Good luck.

 

I have a 528xi and trust me, most of these guys don't provide any helpful info regarding this model other than "get a 535".
I was in the same spot, looking for a bit of performance but not for racing either.
It's certainly a case of what you can afford, and indeed isn't an investment in anyway.

I began with exhaust. Performance wise, RPi exhausts are proven to be good, but due to the cost, I went with a simple muffler delete. Raw engine sound, quite loud though.

I'd also suggest an intake upgrade. I went with AFe and paired it with an RPi scoop.
Another thing worth looking into is ECU chips. You certainly won't find any ridiculous 100 whp gains like the 535s, but you will get a respectable 10-15 whp 10+ torq.
A bms powerbox is also something that could be looked into, remaps your throttle control to give more power at lower depression of the gas pedal.

I'm also playing with suspension, personally throwing on h&r lowering springs (sport) just for the aggressive drop, coilovers for the xi are ridiculously expensive (3k for kWv3s).

Other than that you're left with aesthetics. You can throw on the m body kit as well, certainly looks much better than stock. Will cost a pretty penny though, each part+paint+install unless you DIY.

Lastly, WHEELS!
The stock 528xi offset is 43. I went with a staggered 19in set up, 245/35/19 by 8.5 offset 40 in the front and 275/30/19 by 9.5 offset 45 in the back and it's a perfect fit. You can use tirerack or other websites to see what works for you if you'd like.

Hope this is enough for now. Whether or not there's much more to it than that, not sure since I'm still searching as well, but sharing what I did with you! Hope this helps bud.
Don't be surprised however at the low amount of performance options for us, our cars weren't exactly performance focused, and it definitely isn't as easy as "sell and upgrade to 535"
As most of these guys think,
Enjoy!
Mike

 

Pics of that 528 Sir!

 

Pic of my Bimmer

 

Cheap sound mods include: performance filter ($50) + muffler delete ($150). $200 max
Cheap aesthetic mods include: Angel Eyes ($50) + LEDs ($50)+ roof and trunk spoilers ($200) + front lip ($250). $550 max

I would recommend 19" wheels, summer performance tyres and adjustable coil-over upgrades. These are expensive but the benefits are worth the money.

 

I think I will definitely be upgrading the air filters and possibly look at a sprint booster type mod... maybe look at upgrading the wheels both thats about it... outside of tinting the windows of course. As I said, Im not trying to modify the car into a drag racing machine, I just want to make a few minor upgrades that make sense. Id love to lower the car a bit but I dont want to dish out $1000 for new suspension to do so.

 

Increasing the diameter on your wheels will make your car slower, negating the 10hp gain from a tuner pack, plus take longer to brake - rotational inertia of the weight being farther from the axle center. JSYK.

You can improve stance without lowering by using wheel spacers. I put 10 front and 15 rear on mine but suspect 15 front and 18 rear would fit fine (haven't tried the 15s on the front yet though). That plus bars will be a marked improvement in handling and your ride won't go to hell.

Other than that, cosmetics? Blacked grilles, sport bumper covers?

 

There really isn't much you can do on a NA I6 engine in terms of performance, nothing significant that is worth the money at least. Other than the sound and aesthetic mods, you can improve the handling, which is much more useful for a daily driver. It makes the car much more enjoyable to drive. In this regard, tires and suspension make a world a difference.