I used my TT bike for Outlaw iron distance tri a couple of years ago. Stayed in the position for the 6 hours that it took me to do the course. Not blisteringly fast but it was my first iron distance. Was comfortable all the way round.
in reality it meant I could just sit and spin in comfort in a reasonably aero position. To do 112 miles on the road bike would have been a bit more upright and a bit more weight on the wrists.
i suspect that I was 20+ minutes quicker on the TT bike, but very comfortable and no problems on the run.
i had Retul fitting on both biles.
it was also much less faff than switching saddle position and tri bars on and off the roadie. I used to do that – had a separate seat post with saddle for easy swapping too, but it was still a faff. So I kept the road bike as just that, and had the TT for tris and tri training.
i have since moved to hilly Wales, so the TT bike hasn’t been ridden for a couple of years, so if you fancy a nice carbon Felt my email is in my profile. (Stealth ad alert!).
i did ironman Wales last year on my road bike as it was a hilly twisty course and we had high winds and lots of rain so not TT bike friendly for me.