I have a b16 mani, and tough to go much above 64 mm. - what about polishing out an oem itr manifold - will it go to 70mm or water passage interference probs??

 

plenty, cams & compression, head, etc. the b16a mani is good for low torque but need more topend

 

Bigger TB will give you a little better Throttle Response, but I think the Induction system is more important.

If you go with a 70mm TB go with one tapered to Stock Diameter. This will encourage a Venturi effect.
Much like the J's Racing Intake does.

What kind of Cams are you running? What kind of Compression? Reguardless of how big the TB you engine will only flow as much air as it is setup up for.
to overly simplyfy this if you change nothing about the engine and increase the TB size by say 25% then you will reach WOT 25% earlier. So at 75% throttle your engine is already taking in as much air as possible and there is no increase between 75%-100% Okay I hope this is clear.
However if you have done a good amount of work to the engine o allow it to want to take in a larger volume of air then the above doesn't apply or maybe just to a degree.