This isn't just a fireable offense. Especially given the tendency for computer-related criminal laws to be overly vague, it's entirely possible you could be charged with a crime if you are intentionally trying to DoS your employer's network.
TBH, for most techies I don't think opposition to MITM boxes comes down to "I don't want them to catch me looking at cat photos" but more along the lines of "this will actually reduce security as much as it improves it, and the companies providing these products are also aiding repressive regimes and human rights violations across the globe". Personally, I would find it unethical for the company I work for to buy these products.
Incidentally, "Blue Coat ProxySG 6642" was the only middlebox to get an "A" from the study referenced above. Apparently they didn't test for 1.3...
Then leave the company in protest or convince it not to buy them. DDoSing the company's network is somehow not unethical, I guess?
I need my personal email to do my work. It needs to stay secure from even my own employer. Period.
Collective action (strikes, "work slowly protests" etc.) as a protest against company policy has a long precedent of a) being protected by law and b) being much more effective than a single employee quitting, while simultaneously reducing the downside for employees (in L_\infty norm).
Edit: the old Keynes quote comes to mind: "if you owe the bank $100 you have a problem, but if you owe the bank $100 million the bank has a problem" -- if 1 of the company's devs commits a "fireable offense", he/she has a problem, but if 100 of them do, the company has a problem.
Everything you listed is information that the company already has access to. Why isn't it sufficient for there to be access controls by policy, the same way the company protects other sensitive information from unauthorized acres within the company?