I have lived in the CO / UT area before so I have done this trek probably 25+ times ... most likely one of the few guys on GTAM to have done it on a bike many many times
I have done Seattle to Toronto in 3 days ... that's about 4200km ... so its do-able but based on the per person basis ... I was on an ST1300 but my buddy with me was on a ZX14 ...
If I am not trying to iron-butt the ride across the prairies... USUALLY what I do:
- Day1 - leave work early at around 3pm on Friday ... ride the roughly 700km to Michigan City Indiana. Assuming you keep your dinner stop to a minimum (or don't eat dinner until you get there) then with my Nexus pass at the border I can make Michigan City around 10pm'ish (assuming you hold steady to 120kph while moving) ... this would include 2 fuel stops and you would quickly eat something to keep going at the fuel stop ... and I wear a camelbak for hydration on the road. If you stay at the Super8 off the freeway there is a steakhouse literally on the other side of the parking lot ... I usually eat there since they open until 11pm. Also there is a Walmart 2 mins down the road for you to pick up anything you forgot to pack at home. Since you are doing some riding after sun down I HIGHLY recommend you detour yourself down to Interstate 80 ... might mean an extra 30 mins of ride time but Interstate 80 along Indiana has a "animal alert" system ... its basically a bunch of sensors that have laser beams on the side ofthe highway and if an animal passes the beam an alarm goes off and you have probably a few hundred meters of notice because you'll get alot of flashing lights on the road ... NO JOKES, this works !! I had the lights go off and I slowed down and so did all the cars and up ahead was a deer crossing the road. At least this way you will have some warning that something is up ahead.
day 2 - I would leave really early the following morning ... no later then 6pm since you are crossing through the bottom of the Chicago area along I80 ... I usually am able to make it to Kearney, NE or North Platte, NE with no issues (approximately 1100-1200km). Keep in mind that once you get to the IL to IA border your speed limit will go up ... I believe 65mph goes to 70mph ... 70mph = 112kph so you can safely do about just under 80mph which is 128kph ... Once you get to the IA to NE border your speed limit goes up AGAIN to 75mph which is 120kph ... so you should easily be able to do about 135kph ... Though I'm not advocating it, I usually follow the locals through here and am routinely doing about 150kph ... which sounds fast but keep in mind its only 15mph over the limit

... North Platte is about an extra hour and a bit past Kearney ... so doing a 135kph day go to kearney, doing a 150kph day go to North Platte ... Kearney townwise I believe is 'better' then North Platte from what I remember.
Day 3 - is a leisurely pace of 580km to Denver ... wake up late and leave the hotel by 9am ... get there around 3pm so you can still have time to check out the town. Also unless they have fixed I76 the past 10 years it is the worst road in the sense that there is a pavement break every 2 or 3 seconds ... so its a BUMP-SMOOTH-BUMP-SMOOTH-BUMP ride ... very annoying
sorry for long post, did not notice it was so long until I wrote it