True, most twins can be used bigger. In my opinion, this is mostly because they stay loose also in higher wave riding speeds, even when you carry a lot of volume/width. But when it comes to pure high wind handling, I don't really think twin fin boards behave much differently to singe fins. Then the biggest problem is sheer size, and the board simply getting blown around to much. This has been true for me both with ETs and other twin fin boards I've tried.
But you seem to focus more on the wave riding side of things and the "blown around problem" is not that noticeable until its really windy. Maybe overpowered 4.2 but more when you're on a 3.5. And then I think you can go for a bigger twin fin than you would if it were a single fin board. But you don't have to. Personally, with the ETs I've rather used smaller boards this season since it seems easier to sail in a wave so that you keep speed up on the wave. I used to go for the EVO 80 for light wind, but now I'm using the ET74 or even the 70 (I'm 70kg). But on the other hand, the ET80 is not in my quiver. If I ad that one I probably would have used in the really light days.
Comparing ET70 and E70 is interesting. The E70 (as we know) has an amazing drive and is a super super refined board, especially if you're used to it. I think a lot of the drive comes from the refinement - all the little errors seems to be worked out and there are not situation in which the board hesitates.
Since I got the ET70, I haven't really enocountered a really classic day, with bigger waves. 2m max so I can not say for sure how it compares. So far I feel it has s similar level of drive, but in a surfier way. These are subtleties, but on the E70, I tend to drop in and gain speed and then pin the board down with quite a lot of sail drive. On the ET, I tend to stay up on the wave a fraction longer, go a steeper route down and enter the bottom turn with a more shut off sails and slightly later put down the rail power.
The E70 really wants you to pin the nose down early on and then drive until its time to go very. The ET70 is easier to get on a rail and you don't need to pin it down in the same way. Its easier to keep a really smooth carve with good speed without really needing to put as much power into it on the E70.
But the question then becomes if you _can_ drive it like the E70 and this is what I don't really know yet. So far it has been pretty much faultless in the bottom turning, but I need some bigger/faster/harder/windier conditions to say for sure. Maybe sunday...
Top turns work like you say also on the ET. Of course, there are differences between different twin fins, but my impression is that they all have something special in the top turn in common. In the big picture. ETs really feel very, very much like EVOs but with the twin fin top turn added like you write.
So, one or two boards? I almost spent the whole on this season first on the ET74 and then on the ET70 when got it. I sold my E80 and have a Kode 80 for pure B&J. The ETs, at least when you sail them right, are very good up wind also when only just planing. It strange, because they don't really feel fast and crispy like fx a Kode, but I still still keep planing and go upwind very well (also compared to friends around me).
So, for 4.2 tops, I could pretty much do it with the ET70 or the 74 (but I prefer the more compact feel of the ET70). But 5.7 just would be to much. Maybe on the 74, but even then its gonna bog the board down a fair bit and you would probably be just as effective on a 5.3. If the one board solution is doable on the ET74 will depend on what type of saiing it really is with the 5.7. Do you need it to stay planing or is it just so light that you need it to get out. Do you need the float of a bigger board or do you at least move along most of the time so that 74 will be enough. I really don't believe in something as big as the 80 as the only board. Probably, you _could_ sail it also in the gold conditions, but you would have MUCH more fun on the 70 or 74.
For a two board solution, ET70+80 would of course be GREAT. It will give you lots of healthy overlap in mid sized stuff and mid wind which means you will be able to tune the feel you want from the board in a very good way.
If I would have to only have one board, it would be the ET70 (or EVO 70 which still is an extremely nice board which I KNOW works for anything) and then a good light but powerful 5.3 as my biggest sail.