I've used straight stainless before and it was pretty meh,
The m5 I had in the shed the other day had extensive heat shielding from front all the way to the back and also had the tunnel enclosed. It must be decent stuff and @shrike mentioned. Plus I bet a twin turbo v8 pumps out some heat compared to an NA 4 cylinder.
I have hyper extreme hpc manifold exhaust housing and down pipe, it works better than expected. I'd have to check but it was maybe 130 degrees surface temp at the exhaust ports at idle. On reflection is that good, well I don't really know. But I reckon that's lower than a bare stainless manifold. But it's expensive.