Don't forget HS2 was adopted as a sop to the LibDems to help secure a working majority coalition government. Chinese whispers abound regarding the third Heathrow runway being back on the cards despite all of the bluster from Boris. Regarding Curzon Street being a terminus for the initial stage of HS2 any approach from the south or south east would have to be tunneled or elevated as would any onward extension. I can not think of a worse suggestion for a Birmingham terminus with the exception of New St. New St is hampered by being too central and has restricted access to the station consisting of two tunnels at both ends. Capacity has been reached for years and delays are a way of life.
If, and it is a big if, HS2 goes ahead then another terminus site will have to be found and a mass transit system put into place to connect BHX and New St. East of the city centre has the best locations and motorway access from all directions and would be closer to BHX. The airport does not need another station it has one connected to the national rail system with three fast trains an hour taking about 75 minutes to London. As for the WCML being at full capacity in 2020, I would suggest that is just scaremongering by the proponents of HS2. I have driven trains over the WCML for about 15 years now and the capacity issues are more down to bully boy tactics by Beardies bunch over timetabling, rather than actual capacity issues.