Two hours can be a tight connect in IAH.
If BA is on time. I am 100% this year with BA delays creating a mis-connect every time via the USA.
Passport control at IAH can take time. Then wait for bags, recheck these, and go back through TSA screening. I suggest your flight may be close to boarding by the time you complete this.
If you are flying business you can use Polaris. It is not too far across to the other side from where Air NZ departs.