No way does any transmission absorb any certain percentage of an engine's power. If that was true a 500 HP engine would lose 100 HP through a TH400, not even close. The TH400 takes at the most about 38 HP to drive it at highway speeds, the TH350 takes about 30 and the 2-speed Powerglide takes close to 20.
The 700R4 is closely related to the TH350, not the TH400 so the power to drive it would be similar to the TH350.
The only people who should be truly concerned with a few HP difference in parasitic losses through the tranny are class racers who need every ounce of power to be competitive. These racers favor the TH350 and TH200 since they don't use much power. These race-built trannys usually cost a LOT of dough with all of the special lightweight parts used to cut HP losses.
Just use whatever trans has the proper gearing, fits your car and is the most durable or the best compromise between these three things. The TH400 is by far the most durable for the least amount of $$, no way would I give up strength and reliability in my performance car to save a few HP that won't even be noticed anyway.
