i'm sure you're read this line a million times because i know i've stated it at least a million times
there is no way we can tell you how much HP you'll get from it.
it really depends on your other mods, sorry just annoys the shit out of me when i see people ask that.
you can bore out your stock b16 mani, but i woudln't, i would get skunk2 or itr, bore those out, they have better runners.. or if you're going 70mm TB you must have some wicked cams in there, then i'd look at the JG victor X mani, i think that one takes a 70mm tb without boring.