While I don’t use HR much, I do think about the implications of the higher HR on stress and TTE. I’m a big user of caffeine for racing or for hard training efforts. For responders, the data is pretty clear that it’s a significant (and legal) performance boost and my experience matches that. But, I don’t think the performance is “free” when it’s cranking the HR up significantly even if RPE is reduced. I don’t worry about it if I’m doing a 3-4 hour race or training session, but many of my races are 5+ hours. Is loading up on caffeine at the start of an 8+ hour race going to help when it’s pushing the HR up early or is that just making you stronger early at the cost of fading faster? I’ve done a little googling on the subject and didn’t find anything useful, would be an interesting study if there isn’t already something out there.
I still use caffeine for all racing even though I suspect there must me a cost to the higher HR. If the race is under ~4 hours, I’ll load up on ~400mg before the start and I’ll be hitting the rev limiter at the start. If it’s a long race, I’ll do 100mg at the start and then ~50mg per an hour to keep it a bit more mellow.