We can see from the pressure vs flow chart that the Xirex Stealth is certainly more restrictive than the XSPC X20 Delta that it was tested against. That being said, the Stealth managed to perform better than I'd expected when we ran the actual cooling loop testing.  If Xirex were to include some micro-channels in the base of the Stealth and relieve some of the restriction present, we may see a significant reduction in the defecit between the two blocks. Regardless, the Xirex Stealth block performs quite well as it is, but just remember to factor in its restrictive nature when designing your loop by looking at the beefier Laing DDC Ultra instead of the Pro.