would look awesome though.
not if it was butter...
having bazillions of displacements wont put huge strain on a pc, just the mapper. Displacements are very easy for the engine to render. so from that point of view its very posible.
About snipers... If you have a sniper friendly looking map like this one is suggested to be carefull considerations can be made to make sure it isnt that sniper friendly.
Done by an experienced mapper who can take the time to impliment some sniper handicaps so that snipers are used in the proper proportions would make this an excelent looking map.
Vertical gameplay: sucks
Reason: players dont look up or down. They said in portal how hard it was to get players to look up for spots to fling from... For this reason most players will probably get pissed off at how enemies above them or a long distance from the cliff on the ground are able to shoot them whilst they can't seem to see them.
Having said that, an esperienced mapper could use various techniques to minimise the frustration of having to go straight up.