an RPE of 8/10 is different than it would be at the beginning (eg it might actually be a 7/10 done at the beginning of a workout). If you use power, the wattage for that 8/10 is the same no matter what (ie it doesn't account for fatigue). This, I find, is excellent to push you harder, but you really can't cheat or not be physically ready for the workout o/w you do have to turn down power considerably as the workout goes on.
Another novice type of question: if one is tired for whatever reason, would a 1-2 x 20 sub-threshold (just below) workout be of any benefit (same question as the original I suppose)?
As for converting RPE into wattage, you really should be able to complete all the intervals at the same wattage throughout the workout. If you are fading significantly through the workout, you are setting your initial numbers too high. For an arbitrary example: if you are starting at 400 watts for the first interval, but as the workout goes on you fade to 360 or 370 you should really be starting at 380 or so. Most successful interval sets start out feeling slightly easy and by the final interval are difficult, but not extremely difficult, to complete.
Sub maximal/threshold work is quite effective at building aerobic conditioning. Sweet Spot Training is designed around the idea of pushing around 90-95% of threshold. This allows you to do a lot of very quality work without the extra fatigue associated with training at or above your FTP.