If you are worried about lag then save up for an RB25 engine swap, the RB20 has quite a reputation as being somewhat laggy.
What you are saying is correct, the smaller the exhaust housing (generally) the better the response, but at the cost of top end power.
In any event, none of the turbos you suggest will be very laggy, and I would suggest using the OP6. The rb20 turbo will limit you to about 170ish rwkw, the RB25 to 190ish and the OP6 to about 200. All on about 13psi, so pick the power you want and go for it.
Just be prepared to get teh power bug and want to upgrade again.
Good luck.