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This function is already part of SabNZB I believe and I feel it would be a valuable feature for UE.

As it is now, when downloading large archives (like a 50GB movie or game), it takes a long time to download, then PAR check and then unRar the archive. These functions can be done in parallel.

As each RAR file of an archive is downloaded it can immediately be PAR checked to be functional and unRARed. The PAR check and unRAR process will then pause until the next RAR file is downloaded. This way, upon download completion the archive will then almost immediately be both PAR checked and unRARed, ready to be used.
When a faulty RAR file is detected the unRAR process will be halted until sufficient RAR and PAR files are available to repair the archive upon which the unRAR process will resume.

As it is now, of course, after download completion we then have to wait for the PAR check and then the lengthy unRAR and that leaves a lot of SSD performance and CPU time unused.

Does it work for .7z files too?

I could try to check it, it would keep the unrar task slot occupied then during the whole download (so would add one more thread).

Very likely there is no waiting option in the unrar command line executable, so such a feature would have to use the embedded unrar code. Others use unrar command line dll I guess, but I didn't want to have a third party dll with potentially different versions running in the UE address space with the ability to crash the whole thing or worse to corrupt the address space with culprit unknown, from other side if the unrar command line executable crashes its process will just end resulting in an error message shown for the unpack entry.

I can't say if it works for 7z or regular Zip files, but the vast majority of large archives is RAR.
I suppose you'd only need one thread to do this stepped download-Par-RAR operation for very large archives.

It is a possibility to add dynamic unrar, it could be in v7.5. In the beginning of December I switched to upgrading OpenSSL which was released in early January, then due to exceptional circumstances the work stalled, so first I need to finish where I left off (newznab interface, maybe 75% done) which will be v7.4.4, then it could be next in line, at least it looks tempting.