I am assuming that you're operating a large transport jet (a Boeing perhaps?). If you don't have runway analysis to cover a Flap 5 takeoff with reduced thrust, you have to ask yourself "what if I have a bad day and lose an engine at a critical moment?" Can you be sure the aircraft will perform properly in that case? Yes you may have full thrust available from the engine(s), but the takeoff with reduced thrust changes the departure path and may in fact result in a liftoff point which is too far down the runway to permit a safe climbout. Then you have to consider the regulatory implications. Depending on where your operation is registered, attempting that takeoff without valid performance figures for those conditions may in fact be a violation of the regulations.
So, unless you're a factory test pilot (and paid accordingly), I'd stick to performing takeoffs with valid data.
JMHO