Its a bit hard to say since in high wind wave riding at our spots, not only does the wave get bigger but it often becomes very choppy and generally difficult to sail too. The ET 80 will for sure handle a fast top turn easier than the E80 due to the narrower tail if nothing else. But in these choppy high wind conditions setting up a good bottom turn is very difficult and I think a bigger board (ET80 vs E74) will not make it easier. But all these things are very sailor dependant and also a matter of getting to know ones board and how it is best sailed. So my answer is that I'm not sure how you will find it. Maybe it will replace the 74, maybe not.
See also this thread: http://www.star-board.com/forum/showthread.php?t=4329
If you want to take a chance, I would trade the E80 for an ET80 and then keep the E74 for a while until you know. There is probably not so much money left in an E74 (though by all means its still a very good board by any standards).