the best race I ever had was when I forgot to bring my Garmin with me, I fretted and was trying to borrow a basic Timex or similar but to no avail.
I swam the swim a looked at the race clock when I got out of the water.
Rode the first lap of the bike then the second, again looked at the race clock.... damn, I was 20 min ahead of my expectations.
ran the run, as fast as I dared to, allowing the possibility that I may fail to keep it up, but prepared to back off a tad if necessary.
Finished with a 70 min PB.

since then, I'll sometimes race with a hrm, but leave the timer off or nothing at all.

btw, I have a 305 & 500 and in the one adventure race that I wore the 305 in, the battery died at 13 hrs.

but that my friend, is not an answer to your question.