What is acceptable in terms of power loss for aero gains?
No point having the most aero position if you can't actually pedal it! I recently narrowed my bars 2cm and flattened them a little. It took little more than 15 minutes to work out that this was never going to work; I could barely ride on the aerobars at all. This despite having ridden a 25 in the previous position with zero discomfort a week earlier. Now they are back to slightly wider and angled up (but still UCI 10cm compliant). You can trade a degree of discomfort for time in a 10, but nothing longer.
The TT position is meant to rotate the body whilst preserving the relative position of the rider compared with their road position. The 5cm UCI saddle setback to bottom bracket rule, means that a shorter saddle is normally required. Also, traditional road frames don't have a steep enough seat tube to get to this position properly - my TT bike has 73 degree seat tube so I have a Thompson laid back seat post rotated 180 degrees to accomplish this (and an Fizik Ares saddle to satisfy the 5cm rule).
Often the compromise/default solution is road bike saddle position, maybe slammed forward on the rails with low TT bars. In my experience, this is a recipe for discomfort and inability to apply power because you have not rotated your position, you have just leaned over further to reach the bars - hence power down.
Your position looks pretty good. Can you move the saddle up and forwards? That will open up the hips more akin to your road position. It will also lower your head more to increase aero, so you need to tilt your head up for vision - I always have to look over my glasses and under the lip of my helmet.
My power on the TT bike is really little different to the road bike. Will see tonight at the first Hillingdon ten-and-a-bit