I’ve done a full IM at .82 IF (300 TSS), and a half at .92 (187 TSS). Didn’t walk any of the runs and they certainly weren’t spectacular (3:19 and 1:27), but based off my training splits I don’t know that I could’ve done much better if I had eased off the bike pace a bunch. Running mileage/training had been hampered by injury, which was a bigger factor imho. With that said, I have a solid background in endurance cycling…so being on the bike for 4.5hrs and/or going pretty darn hard for 2hrs was kind of a regular weekly thing. YMMV
With that said, I don’t know how much I agree with the post above me. Doing your long rides at a higher intensity than race pace (and potentially also incorporating some sort of brick to get used to running off a long bike) would be CRUSHING. I can’t go out and do rides at .8+ IF for 3-4hrs and then accomplish anything productive for 2+ days after. I don’t know if I even could do that at the end of a long week of IM training.