Many of us do not like BMW M going from NA to FI. But the thing most of us dislike is that M is no more going to build any special engines but just take the BMW engines and mode them. The best example is the X6 xDrive50i and the X6 M. We shall have exactly the same thing with the 550i F10 and M5 F10.
So why to take an M5 over a 550i?
1. You can mod a 550i for less than the price of a new M5, but you will have risks voiding your waranty.
2. Will it really cost less? You will surely not take a 550i with out any options? If you take some, it very quickly cost nearly as much as an M5, whereas the M5 has already very many options standard.
3. Will you really be able to tune the engine so as BMW M does? You maybe do not see it, X6 xDrive50i 4.4l V8 TT 407 hp / 600 Nm (N63) and X6 M 4.4l V8 TT 555 hp / 680 Nm (S63), but the engine tune is not only an ECU tune as you would do it to an X6 xDrive50i to get 470 hp / 680 Nm. It is much more complex, all the internals are changed. How much will it cost you to do to a standard N63?
4. BMW M is more then just engine tune, as it is, maybe now was, the case for Mercedes AMG. BMW M is on lightweight building. How will you build a CF roof, hood, lid and so. BMW M is aswell all about drivetrain. Stock M3 E92 handles better than stock 335i E92, and if you put KW V3 on the 335i, the M3 with KW V3 will still handle better. Technically there is a huge difference between X6 xDrive50i and X6 M (don't ask me I never tried any of them), but they are not real performance cars. So I do not need to say what a difference there will be between the 550i and M5, no 550i will come close to an M5. BMW M is more than straight-line perfromance.
Here are some examples of the N63 and S63 tuning on X6 xDrive and X6 M by Noelle Motors:
X6 xDrive50i:
Stock:
407 HP/5500 & 600 Nm/1750 (V-Max: 250 km/h)
Stage I: ECU
468 HP/5600 & 683 Nm/3000 (V-Max: 278 km/h)
Stage II: ECU, Turbo
507 HP/5600 & 710 Nm/3000 (V-Max: 288 km/h)
Stage III: ECU, Cat
515 HP/5600 & 710 Nm/3000 (V-Max: 290 km/h)
Stage IV: ECU, Turbo, Cat, Pistons
560 HP/5800 & 750 Nm/3000 (V-Max: 296 km/h)
Stage V: ECU, Turbo, Cat, Pistons, Manifold
650 HP/6250 & 770 Nm/2500 (V-Max: 312 km/h)
X6 M:
Stock:
555 HP/6000 & 680 Nm/1500 (V-Max: 280 km/h)
Stage I: ECU, Cat
671 HP/6250 & 830 Nm/2500 (V-Max: 312 km/h)
So you can clearly see how much the S63 is modified compared to the N63. It is very hard for the N63 to get to a only reprogramed and catless S63. Is N63 Stage IV more relaible than stock S63? I don't think so. After seeing this, imagine the M5 F01, which will be about 400 kg lighter!! And what about the next M3 F30 with its TT L6!!
For information about the Noelle Motors Tuning look at
http://www.noellemotors.de/BMW_X6_E71.505.0.html