No real difference for frequency/phase response, but filtering with more precision allows the filter to have better time-domain characteristics–or rather, to express its time-domain behavior more precisely.
Convolving DSD at DSD rates is the domain of HQPlayer for now. We’ll get faster, but it’s a very demanding use case, so hard to make guarantees about where that will end up.
I’m 95% sure it could work without the 176.4/192 if you could make REW generate native filters at 352.8kHz/384kHz with the same order. I know it doesn’t do that today–but maybe it’s worth bugging them about it–it’s probably not very difficult to support a couple of more resolutions in the dropdown. The process of generating the filters is identical regardless of sample rate. I think the filter upsampling/expansion is really killing you here.
These are good ideas. I’ll get them in the queue.