The EVO 80 is awesome and I can't count how many times I've recomended it to people that have come back super happy and likewise I cant count how many good waves I had on it myself.
But I really think that if you get a Quad the E80 will feel a little boring and will not be used much. So while more than one board is often nice, the risk is you end up wanting a smaller Quad too. And since you make a good argument about only having one board I would actually recommend getting a Quad 81. It's a bit narrower in the tail than the E80, but you still have the same volume for "pure sinking" and the fin power of the Quads more than make up for the missing tail width when it comes to both planing, upwind and keeping speed in the bottom turn (which may in fact be THE strong point of the Quads).
So, now I made your decision harder, right? And maybe robbed Starboard of a chance of selling you and extra Quad76 later.
The thing that would tip the scales over towards getting a Quad 86 is if you actually already feel the EVO 80 is on the small side for the sailing you do that is the most important to you. Then the E80+Q86 option would make a lot of sense. And after all, the introduction of Quads does not make the E80 ANY less good objectively speaking. And even if you would feel that way (relatively speaking) the Q86 will probably handle 4.7 super good too so you wouldn't "have to" revert to the E80 that many days.
Clearer now? Or just more confused?