I dont know about the "purist" part, and IMHO anything needing better then 5uS accuracy is not well suited for interrupts.
Now I understand your numbers, 300 is really 300 iterations of the loop. so its as accurate as the loop takes (given 1000uS pulse, 3.333uS loop time)

I propose you can achieve 1uS with my suggestion, maybe better. But if you can get the XOR to be 2-3 instructions, 2 jumps = 4more (to the grab time and back), then a couple of instructions to save the timer count and port states. You will be right around 12 instructions I think. So not much better then the pulsein, but nicer in the respect you can get all 8 at once.

I am assuming you are using this to check the speed of the 8 uP's. something else to consider :
If they are sending the pulses every 50 mS, you could measure the low pluse (50mS long) instead of the high pulse. This will give you a much bigger window to check the the duration against. 1% is 500 uS in this case. so being off by 3 or 4 uS would then be peanuts.