Stockish engine, use TTY because they clearly work well.  Significantly beyond stock engine use ARP.  That would be my thoughts.

When increasing cylinder pressures you are putting the hardware out of its designed range and therefore are at greater risk of failure. 
If you hit the yield point you are at the point where the hardware is nearing its plastic deformation point, and you actually have a very low overhead before that hardware will stretch and never return.
Below is a tensile pull test graph and corresponding data, the spot marked with the red x is the yield point, and you can see that the test specimen starts to rapidly elongate with the continued application of stress (increasing tension)

Will the cylinder pressures be great enough to exceed the pre set tension of the hardware? Not sure, but with increasing power that is something that is largely possible. 

Long story short, ARP's are not that expensive, and are a very proven and relied on product. 
For peace of mind and potentially avoiding failures I would just run ARP hardware.

jc43089 said:

My thought is why would they use TTY bolts if regular ones would work?  Doesn't the stretching create a stronger clamp to resist the compression?

Also OE TTY bolts are super cheap.

Questioning this myself. The internet insists that you need studs because boost.

I think the advantage of TTY has more to do with assembly and consistency than clamping force (process windows bruh), but maybe I'm mistaken...

Also OE TTY bolts are super cheap.

snap said:

My opinion is that ARP hardware is worth the extra cost since a large investment into R&D has been made to fine tune the metallurgy, design, and processing for a good balance of tensile strength, toughness, fatigue and corrosion resistance, along with specific torque values and lubricant. 

At the end of the day a bolt is a bolt and it will clamp a load as long as the sum total of all of the applied stresses stay within the limits of the molecular bonds.

I see nothing wrong with using 12.9's but if I were doing so, I might do a quick experiment in a test jig with a torque wrench to see what the maximum torque is before yield or breakage is and compare that to the BMW recommended torque spec just to make sure you have enough head-room for thermal stresses and cylinder compressive forces. 

Thanks.

The yield test is something I thought of, but I wanted to do it to one of the stock TTY bolts to see what torque value they yield at to get an idea what torque value I should start at for a non-TTY bolt.

I'm an electrical guy, not mechanical, so this is all a bit outside my wheelhouse... The more I think about it, the more complicated it seems.

At the end of the day a bolt is a bolt and it will clamp a load as long as the sum total of all of the applied stresses stay within the limits of the molecular bonds.

I see nothing wrong with using 12.9's but if I were doing so, I might do a quick experiment in a test jig with a torque wrench to see what the maximum torque is before yield or breakage is and compare that to the BMW recommended torque spec just to make sure you have enough head-room for thermal stresses and cylinder compressive forces.