Editing Terrain Topology at Runtime

I'm about to start working on a R&D project where the player will navigate a certain area the topology of which is defined by a set of data points. This can potentially be huge so I'm thinking of having a grid of which the topology is updated depending on the users position...

I've had a quick scout and have worked out I can modify the Terrains height within Unity at runtime but how resource hungry is this? Would it be better to import a mesh and alter the vertices instead? We'd ideally like this to eventually run on a tablet so it's got to be as optimized as possible. Open to any advice anyone can throw in here...

Thanks guys,

Matt