Hello smiticans,
To answer your question, the only real substantive differences between HIK and LTS firmware is the H.264+ feature is not included for LTS and LTS defaults some of the picture settings (such as brightness, contrast, sharpness, ect) to different (non-50) values.
From a hardware prospective the following is true:
DS-2CD2142FWD-I(W)(S) = CMIP7442W-28M
DS-2CD2042WD-I = CMIP8242W
So the LTS "IPC4" firmware should have worked on the DS-2CD2042WD-I. I had not realized LTS had come out with a v5.3.6 version yet so after I saw your message I tried loading on some of the Hikvision hardware equivalents. Sure enough, it looks like LTS has built some protection in to their v5.3.6 to prevent it from loading on non-LTS hardware. I hope they didn't do this based on my WDR post and findings last year!
In any event, I have been pretty happy now with Hikvision's v5.3.6 build 151105 firmware and WDR performance. I agree with you that WDR in v5.3.3 was totally unacceptable. So my recommendation would be to go with Hikvision's v5.3.6 on Hikvision WDR 2-series cameras.
Hope this helps and let us know if WDR works for you in the Hikvision firmware.