That's a very pertinent observation, Nomi. And I've been agonising over it quite a bit. You're right, of course...the Phantom IS very tall, and its COG must have shifted considerably higher, in comparison to a stock FJ40. However, while it has grown vertically (15-16 inches), it has ALSO widened considerably (+20 inches). The fact that it's also been lengthened (by almost 40 inches) is also likely to affect its behaviour, when negotiating such obstacles as you've mentioned. The approach and departure angles have gone up as well (and are in the 75 and 60 degree range, front and rear).
All of this makes for "uncharted territory"...especially for a completely non-knowledgeable person like myself. I think the Phantom's slope-negotiating capabilities are going to have to be quantified empirically...the old-fashioned way...by lugging it up and down such slopes...in reverse and in forward...and sideways!
All of this makes having a sturdy roll-cage ALL the more important PRIOR to the test & adjustment phase!