MichealDelaney said:
The reason to not open it past 66+ mm is the effect on parth throttle street driving (even with a taper on the bore and good ignition mapping) , since the bigger bore just kills flow speed at the lower rpms. If you want a sluggish laggy engine in part throttle go ahead and bore it to 70mm+ if you like.65 is plenty on the bore and 62 isn't a choke job up top either. A couple of idiots I knew back in the day even got an Infiniti Q45 TB (as I recall it was somewhere in the 75mm bore ballpark) onto a B series thinking like simpletons that if a little more must be good then bigger must be better. Man did they look like fools then. You're welcome to repeat their charade for your generation.
Thats just what i found on a quick search on TI. Another reason to keep it the same is since charge piping is generally between 2 and 2.5 inches to promote good throttle response, and the stock throttle body is already larger then that, there is no bottleneck to get rid of, so no reason to go bigger. Unless you were running very high boost levels a larger unit wouldn't make a bit of difference.
Also, the inlet on the Skunk2 Pro series intake manifold is only 63.5mm. Having a 70mm TB going to a 63.5mm IM would reduce flow. If you really want a bigger IM, id go with a 64-65mm and bore the manifold to match.