Say you've got a session of 16 x 400 with 60 seconds recovery.
If you were to absolutely nail that session you could do each rep at 66 seconds. How much would it affect the training impact if your rep times were slightly slower around 67 or 68 seconds?
Marginal I know, but feels a lot easier to run 67/68 than 66 and I was wondering would you actually benefit that much more from killing yourself to do 66 each rep.
Just run, baby
Not an expert but the difference is 4 seconds per mile (cumulatively) so probably not much difference from a training perspective. The problem with trying to "nail" 66 seconds each time when it's uncomfortable is that it might result in poor results for the last few reps - or even skipping the last one. At least that's what I've done in the past.
sometimes you need to run goal pace...maybe that's 66/lap for you for whatever distance. in that case running 67s or 68s would be worse for specificity...but fine as long as you compensate by adding volume
Generally you should not race your workouts. Now if 66 is the very best pace you can run these 400s at, then 67-68 would in fact be a good pace.
If it is still a long time until racing season, even slower paces can be appropriate. Only when you are peaking for races you should give your 100% in workouts occasionally.
Remember: 'Train, don't strain'. And do not only think about the performance of this workout but also how fast you can recover for future workouts.
It can be a huge difference. With the first situation it can become a 16 x 400 meter race. That's 4 miles worth of 'racing' in a workout. Going a more appropriate pace will allow you go do another quality workout a couple days later, but nailing the 16 x 400 workout can take away from the quality of your workout a couple days later. It's better to do all A- workouts than a couple A+ workouts and a couple C+ workouts. Save that extra couple percent of effort for the race. Also, going too hard too early in the training cycle will accelerate your fitness improvement too fast and you will peak too early and your peak won't be as high as it could have been with the proper buildup.