Mixing Ultegra or 105 left crankarm with Shimano Sora support
Almost every Sora crankset from the past six years will be compatible with either a 105 or Ultegra non-driveside crankarm. To check if your crankset is compatible, you need to see if it's a hollowtech II model. These have the 24mm spindle attached to the driveside crankarm, while the non-driveside arm is then mounted using two bolts and a cap on the end of the spindle once the crankset has been put into the bike.

The Shimano Hollowtech II page here should help you identify whether your crankset is Hollowtech II. If it is, then you're safe to order that power meter, because it will fit just fine.