Typical rise is around 2-4 times nominal. Usually higher than 2. Usually lower than 4. So lets call it 3x rise. Meaning to say than although it is wired at 1 ohm (nominal) the impedance changes alot while playing music (reactive) and its usually higher than the starting (nominal) impedance, hence its called rise.
So if you have say 3 times reactive rise. Your amp is seeing 3 times 1ohm = 3 ohm. Thats likely why you dont see much voltage drop, because at 3 ohm your prolly only sending a true say 800 watts.
Now all of that is very toungue in cheek. Actual impedance will vary with alot of different variables especially frequency.
That only way to see what kinda power your doing is by doing a clamp test.. which involves measuring the output voltage with a dmm and measuring the output current with an ac clamp meter. And even then thw results will off because the voltage and current are out of phase, but it will give you a rough ballpark idea of realtime power.
Long story short, how much power your amp is doing doesnt really work like that..