Does anyone know what would happen? Does the ballast limit the power? If the T5 HO lamps were less expensive than T5 non-HO lamps, would it be ok to just use them in a non-HO luminaire?
HO bulbs prefer more power than regular bulbs. They will work on a normal ballast, but won't give peak performance. A normal bulb in an HO ballast will be overdriven to its destruction.
What, you say? How can this be? You are accustomed to constant-voltage devices - plug a nightlight into a 20A circuit, no problem. Fluorescent tubes are current devices - like LED chips but even moreso. They are non-linear and do not act like a resistor. They act more like a dead short and will draw unlimited current. They need external ballasts to limit their current to spec. The ballast will push through spec current whether the tube can handle it or not!