You could still potentially transfer into a US school,but would likely have to start med school all over, which probably doesn't sound too palatable to you. It honestly will be very hard to get cardiology as an IMG, simply because there are so many US allopathic grads who also want cardiology.
As an IMG, it would be helpful to have USMLE scores at least the national average (but the higher the better). You better get yourself some research as well...I mean get something published. The main thing, though, is to get yourself into the best internal medicine residency that you can get into, even if it's in some state where you don't want to live, some place that is dark and cold, etc. What you want is residency at a good university medical center that has a cardiology fellowship. The better known the hospital is for its internal medicine dept. and cardiology division, the better for you. If you can't get into residency at a major university hospital, then go to a good affiliated hospital, or to a community hospital/private hospital that is known for cardiology.
There are a few US IMGs in cardiology. I know of one attending at one of the teaching hospitals where I rotated who went to St George U., and another one went to an Irish med school even though I think he was/is a US citizen.