Maybe I am wrong, but in a RTwP game that pauses after every round or turn, wouldn't all the characters and enemies be acting simultaneously? For example, six seconds of mayhem wherein everyone on screen is either moving, attack, casting, etc? If I am right, then no, it would not be like TB at all.
Well, Frozen Synapse call itself simultaneous turn-based. I think "turn" refers to player being able to perform or set up certain amount of actions - whenever players can do their turns at the same time, and if the result of their actions will be performed in another phase isn't relevant to the game being considered turn-based. Still, BG2 system doesn't really allow to cue activities, and has other limitation at to what player can "program" - so playing it Frozen-Synapse-like doesn't allow for full interaction with the action economy. It also, unlike FS isn't designed around predicting enemy moves so it's not good system to use for this type of combat in the first place.
As to what a difference between 6sec RTwP and IgoUgo Turn-based - those are different rulesets - the difference should be self explanatory.