RPE, breathing, etc. are great, perhaps the best measures of the internal cost of a given effort. But if one wants to see a number displayed and track that data with a device, that's not really much of an option yet. Garmins will display breathing rate, trying to use HRV data via a chest strap to indirectly estimate breathing rate, but it's only semi-accurate. It seems ok for low level (Z2 efforts), but anything much harder than that and the accuracy seems pretty bad.