Ive noticed that on the same tune my b20 is getting 5 to 8 mpg less with 87 compared to 93 octane. Im using neptune its pretty much the p75timing map with fuel added to keep it in the mid to low 13s under heavy load. Plugs look great no detonation of any kind and the timing mark is at the top of the bend on the ground strap. What could explain the difference?

 

Your current timing map simply suits the 93 octane better.
So with 87 octane and the same ignition tables used as for 93 oct
Mbt will be down for few percent = less mpg.
Could also be an effect from the different BTU or ehtanol content.