IIRC, when I had a chat with Alex from AKS about this, he said that mapped cars run hotter than spec. Consequently, the 5W-30 oil becomes thinner than it normally would in a stock car. This can lead to damage with bearings/cam follower/etc. Using 5W-40 means the oil is as viscous in a mapped engine as the 5W-30 would be in a stock engine.
Or at least that was my impression of it all.