You're already falling prey to the sales pitch of 100ppm. Go read the fine print.
http://www.xerox.com/digital-printing/latest/X80BR-01U.pdf
Page 16 has this:
Color 800/1000 Presses Productivity/
Print Speeds–AWM
• 80/100 ppm (8.5" x 11"/A4 letter), all weights 55–350 gsm (15 lb Bond to 130 lb Cover)
• 4800/6000 full process color A4 (letter) 4/0 impressions per hour
• 44/50 ppm (11" x 17"/A3–12" x 18"/SRA3), all weights 55–350 gsm (15 lb Bond to 130 lb Cover)
• 2400/3000 (11" x 17"/A3–12" x 18"/SRA3 tabloid) 4/0 impressions per hour and 4800/6800 (8.5" x 11"/A4) IMP
4/0 Means four colors (CMYK) on one side, and nothing (absolutely nothing) on the back.
It doesn't slow down for thickness (gsm) but it DOES slow down for size.
You're going to get 40 12x18's SIDES per minute. Not 100. Which means 20 complete sheets of paper per minute. Your first book is qty 4200 of 18 sheets of paper, so you'll be at roughly 4200 minutes, or 70 hours. Divide by 2 for 2 machines, and you're at 35 hours of both machines running simultaneously to print that job. If you work 8 hour days, you're at 4.5 days just to print. Grind for 12 hours a day and it's done in 3 days. Don't forget time to staple and three knife trim!
Don't get hosed by the sales people. ALL of them quote times for letter size sheets, and no one prints on letter sized sheets.