Interesting to see this thread... After getting into our latest F25, I have to agree, I was a little surprised by the braking, its quite a bit softer than I expected. However, I thought it was just because I was used to driving the X5///M. Also, I agree with others, tire choice is a huge factor, and the runflats on the X3 are junk! I was running Michelin Pilot Super Sports on the X5 and it absolutely helped me feel comfortable driving that thing like it was my M3.
So, I too am searching for brake upgrades for the X3, and I'll definitely be getting out of the runflats as soon as I can. I am hoping to widen the stance with a set of 20" wheels, at which point I'll shoot for the Michelin Pilot Super Sports as well.
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2018 F15 X5d Carbon Black/Nutmeg Individual- Oil Burner Tow Rig
2013 E82 1M3 Carbon Black/Black- S65/DCT Track Toy
2013 F25 X3 35i Alpine White/Chestnut- German American Family Hauler
Past: '11 E91 335is, '11 E90 M3, '10 X5M, '14 328d, '03 330i Dinan3 ZHP, '04 330i ZHP, '12 X3 35i