A B18B and a B20B make just about the same horsepower, so top speed and high-rpm acceleration are just about the same.

A B20B does make more midrange torque, and although the resulting acceleration advantage is slightly compromised by the slightly heavier pistons, it will outpull a B18B in the midrange.

The state of tune tends to make more difference than which motor you've got. I stand by my incorrect, fanatical, useless, and anal valve adjustment technique and specs, and any of my customers would tell you that their cars pull noticeably harder after I tune them that way (and no one has ever burnt or cracked an exhaust valve as a result).

That's why my old B18B could outrun EFI cars that were set up the Green Book way.