I will assume we are only considering the well being of the possible people, not their outward consequences of their existence because that simplifies things and that seems to be implicit here.

Alternative experiment: you have to possibility to make a copy of yourself, who will have living conditions of the same quality as you will. Furthermore, you stay conscious during the process, so the copy won't be "you".... do you agree to the process?

Yes.

And what if the copy would have -40 IQ and much worse living conditions compared to you?

As long as his life will be better than not living, yes. It seems strange to want a being not to exist if ey will enjoy eir life, ceteris paribus.

I think the problem here is that our utility functions (conditional on such thing actually existing) doesn't seem to be consistent when considering copying living entities... (as not creating a copy and killing it later are sometimes identical operations, but they seem to be very different to our intuitions).

I have tentatively bitten the bullet and decided to consider them equivalent. Death is only bad because of the life that otherwise could have been lived.