What new JSON-RPC calls are available with SegWit activation?
As you mentioned, strippedsize was added as a parameter to getblock() that shows the block size excluding witness data.   

txinwitness has been added as value to the response from getrawtransaction() and decoderawtransaction().     

That's all I was able to pick up from the RPC... I suspect looping through the transactions in each block and checking for the txinwitness value could get the number of segwit transactions.