I mean, there isn't really a workflow that goes from DV7 -> DV5, is there? Because of the diff in color space, you'd have to re-encode the HEVC video stream. And if you have access to an encoder that supports IPT, don't you just have access to the DV software suite, such that you'd be able to completely reproduce RPUs and trims that match the frames of the new encode exactly, instead of having to transfer the RPUs from the DV7 encode?
Regardless, to my eye, IPT does produce great quality encodes with better efficiency. So if you take a high bitrate HEVC video stream from a UHD BD and re-encode it with an HEVC encoder that supports IPT, then apply the RPU from the P7 encode, you should be able to produce a pretty good result. It isn't that different than how one might re-encode a UHD BD or BD to a lower bitrate for streaming with x264 or x265, and with IPT you'll be getting better picture quality at a given bitrate.
If you go from DV7 MEL -> DV8.1 without re-encoding the underlying HEVC video stream, then you're losing literally nothing, so that would obviously be better quality than re-encoding to DV5. DV7 BL+MEL+RPU is going to be an equivalent presentation to DV 8.1 BL+RPU.
There's also an argument to be made that the extra 2 bits of color in an FEL represent brightness levels in specular highlights that current consumer display panels can't achieve; they represent values of 1000-4000nits. So if our displays can't display them, what's the point in having the FEL at all? Because it isn't that simple. The problem gets murky in scenarios like the Total Recall example that RESET_9999 shared, where the overall picture quality and fine detail improves with the video data from the FEL recombined into the decoded picture.
So, it seems to me, if a UHD BD has an FEL available and you want to know you're seeing the best possible picture quality (inclusive of the placebo effect), then you'd better watch it as a DV7 FEL encode. If the UHD BD only provides an MEL (or is only available as an HDR10 or HDR10+ release, so you're recreating your own hybrid BL+RPU), then DV8.1 is equivalent to what you're going to get watching DV7 MEL.