Tough to say. I would say the 70 is clearly the better high wind board of the two (75 and 70) despite them in many ways being similar. The 08 70 is a tad bigger than earlier versions and the 07 is the smallest and the one that for me is the most all round. But at your weight the 08 70 is probably the best bet of the bunch. In any sort of powered up conditions the 70 will definitely be big enough and I would say that you can use it with a 5.2 to if you use the 23cm fin you have.
Another reason to recommend the 70 is that is will partner the 80 better and since you like the 80 I think it makes sense to keep that one. At your weight is it s super all round board. A 90 will be a bit more effective in some type of conditions, but you will not get the same total range out of it.
If you really want to get at the increased light wind range, then you need to base your quiver around an EVO 90. It is still a super loose board, its just that you need to avoid sailing overpowered as you can with a smaller EVO. A natural partner for the 90 is the EVO 75 (08) which is still a big step up in high wind control relative the 80. Of course, you _could_ also partner a 90 with a 70, but that will leave the important mid range a bit empty. When a 90 is overpowered you can sail a 70, but you will miss the option to tune your sailing by choosing boards (which is often very nice).
A third (or is it forth) option is to go 70+80+100 (or even 90). But don't forget that the 90/100 more adds float and schlogging comfort than planing effectiveness if 5.8 is the biggest sail.
Regarding the small Evil, its to early to give much away, but essentially I would say its similar to a 08 EVO 70 in control, so yes, it may become an option if you feel "experimental". Personally I love the special feeling a twin fin gives but I have also been just as satisfied sailing single fins the last few (Very Good) days here in Sweden. EVOs are EVOs and just as with most well known board models you can pretty much know what to expect and by now I can reasonably accurately predict how different sailors will find the board. But twin fin boards are new and not so much is known about how they will work in different conditions for different sailors. Many reports are tainted by the novelty factor, I feel. But my preliminary guess is still that twin fin boards will indeed work very well also for many "non-super-expert" sailors. So again, if you feel experiental, aug 15 may be worth waiting for.