Running tappet clearances tighter than spec & increasing performance simply indicates a problem elsewhere to me. Over tightening the intake clearence drops the dynamic compression ratio through nearly the entire rpm range indicating either too much timing or too much compression ratio for the fuel used. Over tightening the exhaust cuts the power stroke short but might produce more power due to better exhaust blowdown. Did you try just tightening one or the other to see which was more helpful?

B18B's out running B20E's would indicate to me that the B20E valve springs are shot. B20E's always go faster with dual springs, the originals just aren't good enough.