Don't get too excited
May was primarily protecting herself and her government from the potential consequences of a vengeful and abusive American justice system not so much protecting McKinnon from that. She's entirely happy to ship people off to face that same terrible system when she doesn't personally fear the consequences.
The McKinnon case was indeed exceptional as May described it; it put her head on the line. May only used the Human Rights Act because she had no other way out of the mess. It must have been particularly galling to have to do that, and she won't want to be put in the same position in future.
What the proposed changes will do is protect the government and future Home Secretaries by allowing blame for bad consequences to be put on the courts and judges; "nothing to do with us, it wasn't our decision to make, our hands were tied". In the Assange case they've already got that as it will be Sweden if anyone is extraditing to America, not Britain. In the alleged Muslim terrorist cases; "fuck 'em, no one cares how badly mistreated they are, they're terrorists".
Anyone who thinks the government or May is acting in the interests of the country or its citizens above their own is mistaken.