For DrZ: http://kanican.livejournal.com/

Kaeko and I did crit testing in patch 1.20. You can see the correlation between adding Crit Power materia and how much the damage from crits increases. One important thing to note is that unlike most other stats, Crit Power is a %increase in total crit damage. You would see the maximum benefit from stacking Crit Power when using attacks that deal high damage (such as Doomspike). So to answer your question, it depends on how much damage your WSs do to the enemy your fighting. If you're doing like 100-200 damage, you'd probably be better off just stacking STR/PIE instead of focusing on Crit Power. Or if you're comfortable with gear swapping, just gear swap crit gear before doing a Blindside Doomspike. The Crit Rate for high level mobs is really low (like 5-8%), so the odds of you landing any of your high damage WSs without Blindside is really low.

Hope this helps,

Seiken~