Can't really say as I haven't fired a comp'd Glock but have heard it does help keep the muzzle down. Makes sense too. The down side though is you get more flash which could be bad in low light.
I'd try to fire one before buying in to it. For the average shooter it's not worth the bother I bet. You don't need it for the 357 Sig.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek