[oldpoppy70]

Given a 25000rpm die grinder (new) can this be used in place of my slow weak unit.?

[dfmiller]

Is it variable speed? Given suitable tooling I could work. HSS bits probably will not last long.
Is the spindle run out suitable for your requirements? Both axially and radially.
Will you Z axis raise it ?
Probably not the best choice but it's it's all you have or it's better than what are using it probably is better than nothing.
Some more details on what machine you have and what you want to accomplish will get better answers than my guesses.
Dave