Appreciate all the comments. Yes with RTO armed and above 90 knots the auto-speedbrkes "should" operate but many operators prefer to manually deploy the speedbrakes just in case. Had a peak in the 737NG FPPM and whilst there is a 4300kg penalty landing for the NG, there is no take off penalty at all if the Auto-Speedbrakes are u/s. Did I read this wrong ?
This all leads me to my next question then - what helps the most on an RTO - brakes (auto/manual), speedbrakes or thrust reversers ?