2.8 has the rep of being the ultimate Cannondale USA aluminum frameset. So stiff it'll beat you like a red-headed stepchild.
So, put the new SRAM stuff on it and go pick a fight with some CF riders.
Sora works, but it's a wall flower group, plain Jane. The 8 speed is still an upgrade for the '86, and will still look like a vintage update and get the attention you expect. Same goes for the 2.8 plus Rival.
Well, when you put it like that, it makes perfect sense. . .