I forgot about this question, so sorry for the delay.
First, just like you I enjoy a healthy overlap between my boards since it allows god fine tuning in a critical sailing range.
Now to the question: is the 2008 EVO 75 and EVO 80 to close to form a good two board quiver?. I think that's a pretty difficult question to answer. No question the boards are different. Thats clear even from only looking at the tail widths. Especially when you're well powered up and the wind is not perfectly sideshore (meaning sail handling in the wave riding will require more attention), the more manageable tail of the 75 will mean a lot. But on the other hand, the shape of the 08 75 is more oriented towards looseness in smaller waves, earlier planing and such things (like the 06 EVO 74, the 07 75 was more "radical"). The 08/07 EVO 80 will still plane just as early (and have the added light wind benefit of more area and volume) but it certainly also rocks in good waves. So in the end, I think the 2008 75 and 80 maybe are too close to from a quiver. They will still offer slightly different things, but unless the am is the REALLY fine tune "mid wind" performance, there will be too much redundancy.
So, whats the options?
1. Base the quiver around the 08 75. I think this board have the same light wind/slow wave range as the 06 74 but still manages to keep some fast wave riding performance from the 2007 version. If you skip the 80 and add the 90 instead you will get something that is a total blast in slow wave riding at lighter winds with the benefit of more float. There will still be overlap between those boards.
2. Base the quiver around the 80. The 80 will be a tad less efficient than the 83 with big sails but it will also in a VERY noticeable way smooth out the wave riding. God complements would be:
2a: 2007 EVO 75 which really is a super smooth wave rider
2b 2008 EVO 70. I know you said no to that one, but it is in fact a bit over 70 liters for 08 and even at your weight it carry 5.3. 4.7 is perfectly within the sweet spot and I think you will be blow away with this one, for sure. The versatility of the EVO 70 is just unbelievable and it will start to work (really well) long before the EVO 80 reaches its limit.