I am facing a similar issue.
I have the ambience white GU10 hue spots that shine their light in at around 160 degrees.
Just removing the "diffuser" will not help you since the light from individual leds will give a lightpattern, not nice.
What I need since I can not remove the current 160 lens is some lens to,put in front to focus the light to the 25 and 15 degrees angle I want. Sounds silly but I am going to my glasses shop for help, probably costing twenty euro per lens or so
What you can do is replace the current lens with the one you want but therefore you need to know how the spotlight is constructed. Are there three 1W powerleds in the Mercedes star construction then you can find a lot of options on Ebay, many lenses are sold seperately. If there are ten or so medium powerleds it will become very hard to focus that light since it is intended to be a diffuse light source.