I personally, with the owners, did two installations of the Atlantic arch. The key is in pre-tensioning the arch by cutting the upper cross members long-ish so the legs are wider than their mounts. You then pull them together when you put them in place using ratchet straps or the like. The instructions from Atlantic Tower have been updated to emphasize this.
The last one was Kent's and I'm sure he can chime in on wobbly-ness. It was very stable when we finished the job.
I'd recommend you do the job yourself so you get it right - I don't think there are any "professional" Atlantic Arch Installers (except me :-)