R. Star wrote:
So there is precedent that a warp speed collision will mess whatever it hits up.
Well, not "whatever." A planet is different than a ship, considerably more mass, a powerful magnetic field, and class M planets are deep inside solar systems with any effects being close to a sun provide.
Even the largest Borg cube is tiny compared to a planet.
Starships have deflector systems to prevent even small particulates away from the ship. if the warp field was destructive in of itself, why not just let the particulates be destroyed by the field? This suggests that the field can't do any damage. The high speed impact of the Enterprise in BoBW would have damaged/destroyed the Borg cube. Given the distance separating the ship and the cube, the Enterprise probably wouldn't have achieved warp speed prior to striking the cube, it would have been traveling at "pre-flash" speeds.