As close to an objective improvement as a sequel can get. The core identity of Motorstorm is kept completely intact, even with an entirely new locale, more nuanced boost mechanics and the inclusion of an additional vehicle class.
Perhaps the most underappreciated addition this game brought, however, was being able to customize the entire field of racers for each race in single player. 1 bike versus 15 big rigs on a track that favours the latter was but the tip of the iceberg in experimentation. There were so many different scenarios you could create for fun, or to challenge yourself.
Handling was much improved over the original too, as well as split routes within the tracks, so every vehicle class has a reasonable chance of winning. A thumping licensed soundtrack and challenging AI cemented Pacific Rift as one of my fondest gaming memories, never mind racing games.