it depends on the makeup of the group and the profit sharing. in general though, i would disagree and say the revenue generated by the electrophysiologist is typically at least close to if not more than the highest interventionalists in a group. of course you may have 5 interventionalists in the one group for every ep, but if you are asking about take home pay than the electrophysiologist will probably bring home more than the average of the interventionalists.
If you are talking about only group structure, I would have to agree with you, however, a great many doctors are not in groups, and I'd have to say that in solo or 2 person physician practices, interventional makes more. They rely less on referrals since interventionalists tend to have a great internal medicine patient population, while EP tend to specialize more. Obviously there are exceptions, and exceptions to the exception, however, from what I've seen, I've found this to be the case.