HPI silver (6543) = 301.85gf/mm (per petitrc)
Ride Red (28013) = 301.33gf/mm (per calibration certificate I have on mine)
Doesn't get much closer in rate.
They might "feel" different or it might be placebo. Who knows, I've never run both back to back.
I've heard people doing comparisons before, but also seen people get confused between the different ride red (TC big bore, not TC PRO MATCH) and different HPI (not RS4 pro) silver!