And to what 'exact' threshold of max block size this will have an real effect to the security ? 2 MB ? 4 MB 10MB ?
This is the more serious Q&A to be formulated.
It starts even at 2 MB. Please look into the original Bitcoin Classic. Gavin knew this was a problem, that's why the original proposal in Classic had additional TX size and sigops limitations (if I remember correctly). The higher you go the easier it becomes to attack via this vector.
Yep - and with BU you just set it to what such a security modeling is proofed to be ok - so even you say it starts with 2MB - > 1.5 MB might be good or even 2.5MB - and with future improvments up to agreed 8MB?
Why the hack promoting 2nd layer gaming first? (see the word game in it ? )