"back pressure" is the real myth here. Traditionally back pressure would mean a resistance to flow, being the size of the exhaust or the bends/turns in the exhaust. This actually has an adverse affect on an engine's performance being that it restricts exhaust flow causing backed up conditions in the cylinder.

However, what you DO want is to enhance the effects of scavenging within the exhaust which deals with how exhaust gas travels from each exhaust port in "pulses" that harmonize together and can be optimized for different power characteristics.

All that to say - Open headers will provide different power characteristics from your engine. With an open header exhaust gases exit the shorter tailpipe much sooner which will give you MORE PEAK HP.
- With the exhaust on you will have power in the lower RPM range, invariably producing more torque lower in the rev range.

I would say running the exhaust would be better since the increase in HP will be minute and in the very highest rpm range (better for sustained high RPMS). You will want the lower end torque for the most important part.. THE LAUNCH