There's alot of 3D Modelers that do or did new units for SH3 - SH5.
Lot's of info and how-to's also.
Wise and kovall have a killer mod for SH3 worthy of SH4 and SH5!
gap and Targor Avelany in SH5 are also a couple people doing eye bugging work.
I never compared vert by vert, the SH4 models with the SH5 models.
But I'd see no reason to re-create some of the assets.
Lot's of new stuff was added to SH5 that I tend to think the Dev's saw what Modders were doing with SH3/4 and did an Official version for SH5.

I can point out a few 'Guns' in SH5 that were first done for SH3.