I checked and Verizon says the IMEI on my Sprint phone is compatible with Verizon. So Verizon says the phone will work. But since it only supports 2 of the 3 LTE bands, will it really work optimally or will it always work less well than the Verizon version of the phone?
That will vary by location. In an area that only uses the missing band you'll obviously have no LTE, an area that has coverage for all three bands your signal will probably be similar or slightly worse then the Verizon phone with all three.
As
@mikedt mentioned the phone has to be SIM unlocked, here is unlocking info for Sprint...
Sprint unlocking policy:
https://www.sprint.com/en/legal/unlocking-your-sprint-device
Sprint Postpaid Devices - Sprint will unlock a device from Sprint's network under the following circumstances:
1. The device is Domestic SIM Unlock capable
2. The device must be or have been active on the Sprint network for a minimum of 50 days
3. Any associated Service Agreement, Installment Billing Agreement, or Lease Agreement has been fulfilled including payment in full of any applicable early termination fees or end-of-lease purchase options. Because leased devices are not owned by the user, the end of lease purchase option must be exercised and paid in full before the Domestic SIM unlock is completed.
4. The associated account is in good standing
5. The device has not been reported as lost or stolen, associated with fraudulent activity, or otherwise flagged as ineligible to be unlocked
For inactive devices, customers must contact Sprint Customer Care to request that their SIM unlock-capable device be unlocked and may have to take steps to perform an over-the-air update to ensure the unlock programming has been relayed to the device. Additional validation may be required to ensure eligibility.
Sprint Prepaid will assist with unlocking a Sprint Forward device under the following circumstances:
1. The device has not been reported as lost or stolen or otherwise flagged as ineligible to be unlocked.
2. The device has been active on the associated account for at least 12 months with the account active at that time.