A former Apple engineer said on Twitter:
"@AriX I have no clue where they got the idea that changing SPE firmware will destroy keys. SPE FW is just a signed blob on iOS System Part"
https://twitter.com/johnhedge/status/699882614212075520
Then Apple seems to confirm it:
"The executives — speaking on background — also explicitly stated that what the FBI is asking for — for it to create a piece of software that allows a brute force password crack to be performed — would also work on newer iPhones with its Secure Enclave chip"
http://techcrunch.com/2016/02/19/apple-executives-say-new-ip...