Yep, I = P ÷ V. Series mod so V is double a single battery. Most use a low cutoff of 3.2v per battery, or 6.4v then for two batteries in series. I expect the mod will have to cut you off sooner than 3.2v though at that power.
If you look at trying to run batteries down to 3.5v each before switching them out:
I = 200 ÷ 7 = 28.6A
You definitely need 30A batteries like Sony vtc4s, and the mod will cut you off before the 3.5v drainage. Given a 90% efficiency I would expect to be changing my batteries in the 3.6-3.7v range.