There are obviously some aircraft types around that produce very heavy wake vortices, although they are not soo big/heavy. The 757 is one example. Does the 787 have some similar "reputation"?
For wake separation purposes, the 757 is classed as a Medium (based on weight), even though it produces vortices comparable to some Heavy types (in the UK it's classed as an Upper Medium for that reason, to differentiate it for wake separation purposes from 737s, A320s, etc).
The 787, on the other hand, already belongs in the Heavy category based on weight, so it gets the same separation behind it as any other Heavy.