Barrier and slot within reduced area on a PCB
The clearance is probably going to be the thing that determines the distance as any bare conductors, so creepage doesn't really matter. Usually slots and barriers are used to decrease the distance between connectors. In the case of high voltage (and exposed conductors) this might be difficult because of the need for air gap separation.

Creepage is used to decrease the spacing between conductors/traces on a PCB, and will not help improve the requirements for an air gap needed between uncoated conductors on a PCB. If the design needs traces between the connectors, then use the gap to buy you a mm or two between traces. If the design does not have the need for close spacing of traces on the PCB, then a gap may not be needed.


Source: http://blog.optimumdesign.com/clearance-and-creepage-rules-for-pcb-assembly

If we have a 5kV voltage being ran into a pcb (I know your voltage is larger but I don't have those numbers on hand), the minimum coated gap would be 14.53mm and the minimum uncoated gap would be 25mm. If I was using coated connectors with no exposed potentials, I could decrease the gap between the connectors to 14.53mm. For internal traces, the gap could be shortened to 11mm between traces. If I needed to run traces closer than 11mm, I could use a gap or groove to do this. 

Gaps or groves are most commonly used in SMPS designs that are space constrained. In larger designs where the air gap between a connector is the highest distance requirement, I don't know that a gap\groove would be useful.