I didn't find clear answers so far on one place so I spent some time decoding bits of information on this forum. In apology letter we got some clues, but not real explanations why HMD can't hold 90Hz even if LCD panel used is proven stable using Qualcomm platform(mobile) at that frequency.
What I found out, is that panel is proven stable at 90Hz only by using data compression technology（Qualcomm FBC 2.0 or VESA DSC 1.1）source . I was also confused why would SW engineer say that they need 2xDP because ANX7530 support DSC on MIPI but has only 2 slices source. So I guess original idea was to use 2xDP on Pimax 8K, to go around limits of the chip, but it was proven not possible(pixel clock(HBR2), and DSC limits).
Why did Pimax made decision to try using 1xDP on 8K, and 2xDP on 8K X is probably to reduce cost, and since ANX7530 can't provide even with 2xDP solution option to have native 2x4K@90 input, because Pixel clock limit of 720MHz and lack of support for DSC on DP input, it was regarded acceptable to make more sacrifices, refresh rate being in 75-85Hz range, and using scaler is a must.
My guess is that Pimax 8K was envisioned much like the 8K X, but due to limitations of current tech, it is decided to split solutions into 2 products. If you wonder why 8K X is postponed to Q2 2018, solution to support native 2x4K@90 is by using DSC on input, Analogix is still developing chip with such feature source.
Also to me is unknown if using DSC(on MIPI) prevents scaler implementation, that can explain why not use 2xDP on 8K to solve 90Hz issue(with 2xANX7530).
To note, DSC 1.2 is part of DP 1.4 standard, so only latest cards are capable of taking advantage of this tech to drive 2x4k@90 on 8K X HMD. Also to note, there is DSC on MIPI and DSC on DP, ANX7530 has DSC encoder for MIPI, don't support DSC on DP input.
I don't blame Pimax for anything, maybe it is unfortunate to find out that without DSC/FBC panels can't operate at 90Hz, but I expected them to start campaign with Prototype V1 that is using 1xDP why even bother with 2xHDMI I am yet to decode . So V3 had to be V1, that is my point, are there too many compromises with final 8K HMD we are yet to find out - when backers get their units.
About myself, former CS student. Not a backer, but interested in VR development, would love to see how Pimax solves some of tech challenges on road to next gen VR.
Maybe I am wrong, because there is also one part that I don't understand in apology letter, it is noted that 3840x2160@80 is native res per eye tested and stable, we know that V3 is using 1xDP and ANX7530 with scaler, so I guess for that testing one screen is only tested with input res of 3840x2160@80(scaler off), but they wouldn't be able to test 90Hz since chip is limited to 720MHz pixel clock, and when transferring 4k@90 pixel clock is 811.44 MHz. So not sure if there is way to test my theory that LCD without DSC on MIPI can't hold 90Hz, or maybe it is just Pixel clock that is limiting factor.
Maybe Pimax didn't want to share this because it would bore backers to death, who knows, I wrote warning (for tech savvy) xD