This one is pretty easy. If you are experiencing NO fps change, then this would be due to your massive resolution causing your GPU to use up as much fill rate as is essentially present. I don't know your system, but you are really stretching it with that resolution and this add-on.
If you "have" to have HDR on then you have to live with what you have. Laminar is not done with optimizing HDR, and until they are, I don't know what else to tell you. It's known there are issues with HDR as a whole. No doubt, it is beautiful at night though!
Here's the skinny, folks:
We're at an interesting point in X-Plane development. We can either stick with status quo, or we can push the boundaries and show others in the MSFS "what's up." There have been some GREAT simulations released over the years, but one of the most common things I hear about these simulated products is that the 3D is either "bad, lacking, sucks, or okay" and the textures are "not very good". We're changing that.
The reality is the Saab has been VERY well received thus far, and we're VERY happy with the sales so far. I have yet to receive an e-mail from any customer about this, and those who have complained are right here in this thread. It's a very small group.
Hardware will continue to get better, and we will continue to be able (collectively as users of X-Plane) to adjust our settings for the better. The MSFS world has been doing this for YEARS. Had we made a sub-par design, hardware could get better, but our design would never change.
I think once Laminar gets around to smoothing out more HDR issues and how it renders, we'll be on a good track and everyone will be happy. There will be a few bad apples, but overall things should be good. The Saab is definitely NOT unusable by any means, especially with people seeing such high speed increases with HDR off. This alone should tell you that the Saab is not the issue, but how HDR renders and is not yet fully optimized is.
I would love nothing more than for HDR to just work, because the Saab has some amazing night lighting with it. For this, setting adjustments may be necessary. I am away from home right now, but when I get back into the office I will definitely test out more settings for HDR and see what I can provide as help for people. If we could control this, we would, but we won't sacrifice the look of a beautiful product that runs without issue and with great frame rates for 99% of people with HDR off.