+1 Adaptation is key but sometimes slow.
I do about 50-60TTs a year, as my stock in trade of racing. I ride it about 60% of my training time winter and summer (although more indoors in the winter!) My power is within 1-2% for less than an hour on the TT bike ( worse at 2hours+). 20% drop sounds rather dramatic! I'd try more flexibility training or raising the bars and progressively lowering them over the next 6months as you adapt to a more aggressive position. It should always be a trade off between power output and drag.
I've had the opportunity to chat with a lot of the top echelon of UK TTers this year (and trained with a few) as I moved to an area which happened to contain the fastest course in the south of the UK. Almost all of them have comparable or higher outputs on the TT bike compared to the roadie, adaptation to the point of sacrifice to road riding.
The biggest areas which impair power outputs are hamstring/gluteal tightness, the constriction of hip abductors, and breathing restriction.
Mine power drops like a stone if my torso angle is lower than 5degrees. I've tried 0degrees and drag reduction was barely noticeable.
I rode this for a 4 hour TT a few moths ago and felt pretty good throughout. (I have no idea why i was barefoot!)