In my listening tests of subs at Axiom (not nearly as elaborate or extended as listening tests of other Axiom speakers), I'd say yes, the 800 and 600 do not differ in a qualitative sense. That is, I don't write down on the rating sheet stuff like, "Oh, it's obviously a sealed sub!" Or "something tells me this is a ported subwoofer".
"Tactile impact" from subwoofers is a product of extension and output capability. The reason Ian made the 800 a sealed design with two drivers (as opposed to the ported 600 with one driver) was to offer deeper bass extension without port noise that would otherwise intrude at very low frequencies from a vented design, and also to increase the overall output.
Remember that with a sealed design, you are "throwing away" all the back energy from the driver, so you have to have larger amplifiers or double up on drivers to achieve the same SPL that could be obtained from a ported single-driver design, other things being equal.
The comparison you mention, with the 400 on top of an 800, is kind of specious, at least in a large room. The 400 was designed for deep bass extension in a small room, so it isn't capable of the same SPL as the larger EP800. If tactile impact were the criterion, the EP800 would win, simply because of its greater output capability.