I've used the Test 400 on and off since 2013, never in any huge doses, but I can say that it's legit. I get itchy nipples even at a mild dose, so the potency likely matches the stated dose. Also I've never had a really painful pin or infection in all that time, which is partly down to proper hygiene practice and partly testament to the gear being made properly.
I can't compare it to the Test E since I haven't tried it, but I don't really see any reason to switch unless you want less frequent pinning.