Will 90" L x 25" W x 1.5" thick butcher block table need additional pair of legs mid span, or is it stiff enough to be ok w beefy legs on ends only? Not concerned about strength of legs, just whether table might bow without an additional set of legs in the middle. Overbuild?
For a 90" long butcher block, only having legs on the ends will be enough to support any weight that you would realistically put on it. A solid wood top that is 1.5" thick and 25" wide will be very, very strong. But...
With a work surface like this, sheer strength is not the only concern. You want the piece to feel very solid. It is a butcher block after all, so you want it to feel very sturdy to compliment its looks. I think that another pair of legs in the middle will be enough to stop any bounce or vibrations you would feel in the top. More legs than that would be more of a design choice. Sure 2 sets of legs in the middle would be super solid, but the single set of legs in the middle will get you very close to that.
In the end, it comes down to your design goals. A fully open span could be possible if you had a thick skirt board on the top and near the bottom of the legs as well, but that extra visual thickness of the top might not be what you want.