I run 35/35 all the time, but am almost always by myself, or a passenger at most (and both light
). The manual refers to a full load, so if that's the case, I'd follow that, but I'd only increase mine if I was going to drive flat out for a while, which won't be the case.
On all my previous cars the TPMS went off between 29 and 30. Haven't tested the M3 yet, but I predict the same, since recommended pressure is 33. Any more and TMPS would be going off all the time in winter.
Finally, the 'flat detection' system mentioned above is to catch a flat before TPMS (they're separate), like when you have the tires inflated in the 40s, I guess.