Lighting – Will high wattage LED bulb work with this light fixture

AFAIK, bulbs with a GU-10 base are driven directly from electrical mains, in which case it's OK to use a lower-power bulb. Your fixture won't reach the temperatures that it's capable of withstanding.

The only place that I know of that it might not be OK is with low-voltage (usually 12V or 24V) lighting systems such as track or task lighting: some transformers have a minimum power output that they like to supply, so if you use lower wattage bulbs, you should use more of them to compensate (somewhat defeating the purpose). If this applies, you would have an entry for Minimum power on the specifications for the lighting system.

Your fixtures should have a maximum thermal power rating, as long as you don't exceed that with your bulbs, there should be no safety problem from the fixture.

If you were to use light bulbs with electronics such as CFL or LED, you would also need to insure you don't exceed their temperature ratings. CFLs, for example, are usually made to be used in a "base down" position so the heat will rise and dissipate. A CFL used in a recessed fixture will be in a "base up" situation where the electronics will be hotter.

Have you considered using a PAR E27? I don't know your fitting size but a PAR20, 30, or 38 will probably work out better than trying to adapt a GU10.