"Are 800, 850 and 900 fills going to offer the same insulating power given the proper amounts of fill to achieve EE's ratings with the only difference being the amount of fill and total finished weight?"
Essentially, yes. The quilts should all be equally warm as they are designed to provide the same loft (i.e., insulation), regardless of fill power.
Technically, though, there will be a few other differences. The most substantial difference between 800-900 fill will be price. Also, the higher fill power down will compress a bit better, though some evidence suggests the lower fill power down will retain it's loft better in wet/humid environments.
In short, if price matters, I'd go with the 800 fill.