You will have local transport networks split across the two sites and huge numbers of people making this awful transit outdoors between the stations. What do you expect the NRT connection already allowance to be? In many cases it will negate the benefit of the higher speed.
The connection is easily possible in 10-15 minutes for most people who are able-bodies and roughly knowing where they are going; most passengers may make an earlier onwards connection than NRE may advertise. I suspect by far the greatest % of arriving passengers will be bound for the city centre anyway, in which case Curzon Street/New St is swings and roundabouts.
It must also be noted that:
-Being right next door to Moor St (in effect the same station) will substantially improve the interchange from the Snow Hill lines (the local transport network is already effectively split), and
-Should the Camp Hill chords be built, that's more connectivity from Moor Street, and
-Classic WCML services to Euston are not going to evaporate for those people that really don't want to make the interchange between the two, and
-Some passengers will have the option of changing at International rather than New Street, using the people mover to get to Interchange for HS2
Again, I would like to understand what a reasonable, practical, affordable and local palatable alternative solution to the Curzon Street site would be. It is a necessary compromise between build-ability and passenger connectivity.