Using 10 bit ADC for only 8 bits on a PIC MCU
This thread:  http://www.edaboard.com/thread155430.html  should help.  Basically, you'll have to do some converting on your own, but configuring the ADC to left justify will give you bits 9-2 in the ADRESH byte, which you can then use as your 8-bit value.

I don't think you can configure the chip to give you 8-bit precision at the cost of higher samples/sec.  That's a design design made when you order the part.  I could be wrong though and I would consult with the appropriate data sheet.