My opinion is that he most certainly would have run faster later in 81, had he some of the advantages that Rudisha had last year; even without giving up the 1500/mile; and had he the desire or incentive to run faster at a distance he'd already put out of sight in early June that year.
We should remember that Rudisha had a gradual build up throughout last season, with several record attempts, getting faster as the season progressed. At the beginning of June he ran 1:42.04, with Kaki chasing him all the way in Oslo. He improved 0.5 secs by mid July, and another 0.5 secs by the end of August. Coe's 1:41.7 was also run at the beginning of June, at the time making him 1.71 sec faster than anyone else in history and 2.5 secs faster than anyone else was to run that year. He'd always intended to have a serious attack on his 1:42.33 record in mid July 81, again in Oslo, but decided to see what sort of shape he was in at Florence, expecting a low 1:43. His father/coach, who attended all his Wr races, didn't go to Florence either, missing what was to be perhaps his son's best run.
In an interview in mid July 81, Coe said, "Although the season started well with that 800m and 1000m world record, the real peak of the summer will come in late August and early September, with the Europa Cup final, and then the World Cup in Rome, plus big international meetings in Zurich, and Brussels". It had been planned all season that he would clash with Ovett in the Golden Mile in Brussels in late August; but Ovett pulled out the week before.
As the above quote implies and common knowledge dictates, an athlete doesn't aim to reach his/her peak in early June. Had Coe had 2 or 3 more carefully paced attempts at the 800m later in the season, instead of chasing the 1500/mile Wrs and winning gold at the Europa & World Cups (much bigger and more important events than they are now), I'm certain he would have gone quite a bit faster. After all, just look at how much better he was in the Brussels Mile compared to the Zurich run 9 days earlier! The more you attempt something the better you get. Look at Komen's efforts at 3000m or Ngeny's attempts at the 1000m. Both needed several attempts to get it right.
It's also worth noting that Rudisha has a personal rabbit who also trains with him 24/7. Tangui is told what to run, and basically sacrifices whatever talent he has at the event, to ensure Rudisha has every chance of running his fastest. That must be a benefit. Moreover, Tangui often takes the pace beyond 400m, usually to around 500/550m. Coe didn't have this in either of his record runs. In fact, Coe's rabbit in Florence, Konchellah, slowed at 400m, and Coe had to run wide for about 30m to pass him on the bend, costing him precious tenths. I would suggest his run was worth nearer 1:41.5 had he run the exact distance, only 0.5 behind the current figures.
Looking at video footage of Coe's 1:41.7 run, the Florence track doesn't look a particularly new or fast one, and certainly the modern "mondo" tracks Rudisha ran on in Berlin & Rieti would be marginally faster. By how much? I don't know. What I do know, is that Coe's run 30 years ago more than stands the test of time and is comparable, given more equal circumstances, to Rudisha's runs last year.
I strongly believe, had someone come along in July 81 and threaten Coe's new record, obliging him to attack it further, that he'd have run 1:41 flat or perhaps faster. He told AW in an interview at the end of 81, that his main aim for 82 was to win the European 800 title and lower the WR to under 1:41, intimating he would experiment in training with a first lap in 48/49secs. Unfortunately he had serious injury problems mid season 82, then glandular fever, followed by persistent illness throughout 83. This prevented him from ever getting into the form to contemplate such an attempt. He then faced a race against time to be ready for LA in 84, before injury curtailed his post Olympic outings, just as he seemed to have returned to something approaching his 81 peak form.
Hindsight is a wonderful thing, but had Coe known in 81 what was in store for him in the years to follow, and the emergence of Cruz in 84, I feel confident he'd have taken advantage of his magnificent form that summer, focussed more on 2 laps and further improved his 800m pb. He may well still be the WR holder at the distance.