Back in the 1970''s I knew a fellow who had cruised the Carribean for quite a few years on a Tartan 27. Also in that same general time frame there was a couple doing a what I have always assumed was a circumnavigation who would periodically write articles on their voyage. I vaguely remember aticles about Pacific Islands and about cruising the Med. Of course, in those days it was a bit more common to have small boats shipped as deck cargo on big ships so they may have been shipping from region to region.
Of course these are now 30 or so year old boats. Still they have always been seen as seaworthy little boats. With careful inspection and upgrades I imagine they would make a good cruiser for spartan distance cruising.
I haven''t heard of anyone doing crossings, but I suppose it would be doable. My first boat was a Tartan 27 which I sailed in New England for a couple of seasons. It''s a good sailing, sturdy boat. A dock cleat broke loose once in early spring during a storm (I was out of the country at the time); while I had to have some work done to repair the bow she actually did much more damage to the dock than was done to her.
If you own a T-27 and you''ve never read "The Coast of Summer" by Anthony Bailey, you should do so. He doesn''t go offshore, but he takes his T-27, "Lochinvar" around New England for a summer of coastal cruising. Still and interesting read, and if you know the boat his descriptions feel...comfortable.
Also "My Old Man and the Sea" by David and Daniel Hayes is about taking a small (25 ft) boat around Cape Horn. Not a Tartan, but similar in stature.