I am wondering if it is worth the money to upgrade my 528i or just trade it it for a 535 or a 550.

 

if you want more power, then get something else. if you just want cosmetic upgrades, then keep it.

 

If you're really looking for more power, NA upgrades won't do a thing. Just trade in for a 535i if you really need that power. Otherwise I find my daily commute only needs the 528xi. My 335i is fun on the summer days where you're just cruising and tunes are extremely cheap and cost-effective compared to any NA upgrade you do.

If anything you may be able to swap the intake manifold + tune from a 530i I think? I believe it was swappable for 530i > 525i, not sure about 528 though. I'll let the others chime in on this if they don't flame you for searching. To be fair, this topic has been covered many times.