you are going to have to vent the box. a 600W will put temps around 102F in that space.(estimation)

to give you an actual example 400w in a 4x4x4 space with no vents hits 96F
a 250CFM fan would do you wonders and put temps around a mangeable and ideal 77F.

I wouldn't bother with the insulation.
Black white plastic is far more convenient as it is scrubbable.

In terms of yield...depends on strain...
To give you an idea, a 4wx4hx8L box, bagseed, 2 x 400WHPS

12/12 from clone - 100 plants yeilds 16oz every 7 weeks
6 weeks of veg - 28plants yields 24oz every 7 weeks

You gonna keep moms?