Just completed the 3x2k intervals on the track with a warm up and cooldown
1st set: 9:29 HR 163
2nd set: 9:31 HR 170
3rd set: 9:27 HR 173
My max HR is prob 188
I do a 5:30 per k for 8-11k at least once a week with a run group.
I wont say it is very comfortable, 6 per k would be very comfortable.