Whoopsy:

Just an educated guess, It's 20 seconds faster than a 600hp Carrera GT, almost 12 seconds slower than the almost 900hp 918.

I was gonna guess 800hp, but from previous laps times we know the GTR's awd system can corners pretty well, and that's where it can make up a bit of time vs comparable cars. 

My final guess is 700-750hp. 

As for the 991 Turbo with 700hp, hard to say. We know the 991 GT3 will have a sub 7:30 time, the 991 Turbo S did one in 7:24 or 7:26 I think. So with an extra 150hp or so, it could gain those extra 16-18 seconds. 

I just wanted to illustrate that even if the Nismo had 700hp during that run, 7'08" is a huge achievement, one you're not able to achieve with massive power alone. I don't think a 991 Turbo (or any other car fwiw) would gain 16 seconds over one lap with a 150hp increase alone.

I'm sure the Nismo is a very talented package, it's far from my cup of tea, but I admire what it achieved.