Good to see you here...
...suburbanmd!
OK, here is my fully-qualified *personal* statement on horizontal vs. tilted performance, the latter of which I will comment on. No marketing literature required.
On my machine, a late 2005(ish) Bosch Nexxt 500 series, I've noticed that its particular combination of drum tilt and paddle design means that on small loads, the back 1/2 of the drum is where the clothes get washed. On medium loads, its probably the back 3/4. When it is jammed full, the whole drum is used, but the pile is thicker in the back.
After spin, small loads are rammed against the back 1/4 of the drum and back plate. On medium loads, about the back 1/2, and full on the back 3/4.
** Of course, the exception to this are my 4-year-old's socks and mini-washcloths which always seem to end up stuck in the door gasket. This does highlight the clear advantage of a tilted drum, namely the ability to open the door and toss things back into the mix that get stuck.
What all of this tells me, again on my particular machine, is that most of the wash action (so to speak) occurs in the back. They wash there, they end up there, and that makes sense since it is on the downhill side of the tilt and that's where the visible water is.
On really full loads, the clothes in the front are pretty much just flopping around on the drum without much / any liquid under them (but obviously on them and the other clothes.) Yes they'll turn over and end up in the back eventually, but their duration in the front is less-productive than their time in the back. Even the water scooped up by the paddles tends towards the back.
Hence, my hypothesis about true HA machines vs. tilted ones. It is a nice theory, at least.
Did that make sense or am I just rambling on again?
;-)