I went this route for several reasons!
1. Official Stilo roof bars. On which I've mounted the same brand Thule reasonably high spec carriers.
2. I didn't want un-necessary hacking of the rear loom to fit a rear mounted carrier as I've not seen a single rear mounted carrier thats not been illegal (other than the high mounted van rear carriers). You've always got some obstruction of the lights or numberplate. Which basically means you need to mount a light bar on the bikes.
3. You loose access to the boot while bikes are mounted on the boot! So roof mounted means no need to roofbox as you've got your entire boot to play with for packing.
It does look quite high but it was no bother (for me anyway) to lift my bike up onto the rails, the lock to the bike frame is a piece of cake to fit with 1 hand while you hold bike stead with the other. As for the height itself, well a transit van is taller, and i've not seen one of them stuck under a low bridge! Plus trees tend to be well pruned by double decker buses so I doubt that's going to be much issue either.
My only concern is that no mater how tight you pull the front wheel strap the wheel is still able to move slightly from side to side.... thank god I don't have a sun roof to watch it lol.
Got my dad coming over one night as he's got Thule carriers on his Volvo so he can check the fitting for me, although to be honest even someone with substandard IQ could fit the Thule carriers in less than 5mins.
Oh and I won't be removing the carriers when I reach my destination as I plumped up the extra cost to b uy the locking version. Locks to your roof rack, and also locks your bike to the carrier (although I wouldn't trust that for a day in a carpark lol), certainly put off the opportunist thief!
My only other concern is muck.... i'll have to take some cleaning cloths along with me to clean the bike wheels before roof mounting them to avoil fall out on the roof and potential damage.
Now i'm scuttling off to check Argo gallery lol