If you need ultra-high ISO then mirrorless is not for you.
That's not because of sensor noise (the Sony A7s would rule them all here) but in terms of AF.
Contrast detection AF is much more accurate than phase-detection AF on DSLRs but this advantage fades towards very low light.
But that depends on the amount of light we are talking about here.
The question of the day is however ... why do you want to upgrade at all ? :-)
That's not because of sensor noise (the Sony A7s would rule them all here) but in terms of AF.
Contrast detection AF is much more accurate than phase-detection AF on DSLRs but this advantage fades towards very low light.
But that depends on the amount of light we are talking about here.
The question of the day is however ... why do you want to upgrade at all ? :-)