Is Schannel supposed to use SHA1 for (EC)DHE params?
I'm led to believe the use of SHA-1 for the ECDHE parameters is not due to misconfiguration, but is rather the intended (albeit not ideal) behaviour of server-side Schannel. This behavior seems to be present in versions up to Windows Server 2012 R2 and even continues with Windows Server 2016 Technical Preview 5. 

I've captured handshakes with many, many random servers that advertise they're using some version of IIS and the negotiated cipher suite uses ECDHE. In all cases, the signature algorithm chosen for ECDHE params is SHA-1. I couldn't find a single IIS server that used SHA-2. 

If anyone has any evidence that IIS/Schannel is capable of using SHA-2 in this scenario, please post, otherwise for now I'm going to consider this answered as "by design".