i was once like u, not too long ago, didn't really know anything but quite willing to learn (just lazy wanna use google =Pp) so let me try to break it down for u, correct me if im wrong sifu-sifu sekalian.
with a 2.0 block (b20), you gain more torque as torque and cc positively correlates. (bigger cc = more torque). the reason ppl use b16/18 head is solely for the vtec function. original b20 doesn't have vtec. its just a DOHC (new one not sure but we're talking about b20b). so matching the b16a head with a b20 block u get both. a vtec head with a powerful 2.0 torque monster block. however, the block isn't made for high rev, unlike b16/b18's, therefore, reliability would be your main issue.
b16a is no doubt a fun engine to drive, but after awhile the pull isn't there. same will go for b20.. hehe. making a b20 reliable? now that would require some well spent cash. to be honest, plonking in a stock b20 block with b16a head, wouldn't do much justice. power wise of course u will gain, but b20's stock piston has such low compression... 9-ish if i'm not mistaken?