Hi, very nice 2 stroke choice. I've had two of them years ago. The center cylinder is an exagerated story. I've have several triples and if you jet them to factory specs or will take the time to make sure none of the cylinders are lean, that bike will give lots of smiles. You'll want to make sure the oil lines are in good shape - I've seen some get brittle and break with time, but those are still available and easy to install.
The GT550 has the 2nd "best" looking engine put into a bike, in my opinion. The GT380 is my all-time favorite looker only becase of it's proportions. Performance-wise, mine were a close match to the CB500 Fours I've had. It is a well mannered 2 stroke, nothing like the Mach III's of the day. Of course, no wheelies each time the tach hits 5,500 rpm..........
Enjoy.
Gordon