Tenure isn't the only difference between FISC and the Article III courts, either. What other court is so limited in the controversies it can hear? FISC judges have warrant powers, but what penalties can they enforce? What are the differences between the powers granted to FISC and the powers Congress already has to compel testimony?
Congress could tomorrow replace all the procedures of FISC with those of the Loyal Order of Water Buffalos and I don't believe the Constitution would have much to say about it --- absent what it says with or without FISA as it pertains to the Fourth Amendment (ie, no matter what rules FISC uses to decide cases, SCOTUS can overrule the law that animates FISC if it decides the process contravenes the Fourth Amendment).
I wish your comment was at the top of this subthread, though.