BigArnie,
I can do about 80% of my 1RM. I start off slightly below parallel. I am a high-bar deep squatter, so this is actually a little high for me. But, any lower and it is really hard to set my back. And yes, it is very hard to break the inertia with heavy weight.
As far as being a subsitute for box squats, I should probably shut my mouth. As I said, I don't really like to perform box squats, so I guess I can't really make a fair comparision without having a lot of experience with the exercise.
Truth be known, I am not really a "hardcore" powerlifter. I don't much care for Westside methods (though I'm not saying there is anything wrong with them), and my routine is actually a hybrid of olympic weightlifting and powerlifting, and I will be adding an "odd object" day this week. I'm really more into overall strength training, rather than one specific event. I do like competetive powerlifting, although I have competed only once last year for the first time (a hand injury prevented me from competing further). My hand is healed up now and I plan on competing at least twice this year, though my training routine will not be 100% powerlifting focused.