Make the assessment match the site
Give the designer the exact address, elevation, exposed sides of the house, heating history, and cold rooms. Describe wind-related drafts and whether the current system struggles. Request the outdoor winter design temperature used in the calculation and why it fits your location. A Denver assumption should not be carried into a mountain home without explanation.
Ask how the assessment accounts for insulation, air leakage, windows, room layout, and the areas you want heated. If the home needs air sealing or insulation, have that work assessed by the appropriate home-performance provider. Ask whether the equipment decision should follow those improvements.
Require an elevation answer from the manufacturer
Request exact indoor and outdoor model numbers, approved elevation limits, and any manufacturer adjustment required for capacity or airflow at the site. We would not apply a blanket altitude percentage from a forum. Have the designer show the applicable manufacturer guidance or obtain written manufacturer confirmation where the published information is unclear.
Ask for a comparison of the calculated heat needed and available equipment output at the selected design condition. Also ask what happens below that condition, during defrost, and if the equipment is unavailable. The plan must name any backup source and how it is controlled.
Check the outdoor location
Have the installer assess wind exposure, snow accumulation, roof runoff, service access, and meltwater drainage against the exact installation instructions. Do not improvise a wind enclosure or cover to protect an operating unit.
We would favor the proposal that explains these details over one that only points to a lowest operating temperature. Being able to operate at a temperature and meeting this home's heat demand are different promises.
Your next action is a site-specific design package with manufacturer-supported elevation information. Until those pieces are clear, keep the existing reliable heat plan in place rather than assuming a proposed heat pump can replace all of it.