I always regress to the 1g/watt standard of grow efficiency to get an idea of what an X hundred watt MH/HPS will yield. Given the tent dimensions you're likely better off with 2 x 600w, but a single 1000w isn't bad.
Here are my wildly subjective and unreliable (however realistic) estimates:
Assuming 1 x 1000w HPS under a...
Short veg time SOG (dozens): ~600g
Long veg time, 2 trees: 600-900g
NBG-style scrogging (2-4 plants, 5 weeks veg): 700-1000g