Picking out parts for a 9" .300 blackout pistol. Looking at the warcomp for the muzzle. Any reason NOT to do this? As in, is it a waste of money or not beneficial? Anyone else done it?

If you plan on running a Surefire suppressor then sure why not?  If not, I wouldn't IMO.  I had an expensive non-suppressor mount brake/blast shield and quickly realized I should have spent the money elsewhere.  Sold it for a loss.  The muzzle blast of my 9.5" .300 isn't nearly as bad as my 16" 5.56 was with a Surefire Procomp brake.

As always YMMV and certain terms and conditions may apply.  I found that using a brake with a short barrel would pepper my support hand with micro shrapnel or powder, not sure exactly what it was. It didn't hurt but it was annoying.  So I found myself using the blast shield 100% of the time which did the same job a $25 flash can could do. Now I just use a standard A2 until I can save up for a can.

I have a warcomp on my 8" pistol.  I do not notice much blast. I have not shot it with any other muzzle device so that may not mean much. The warcomp seems to be mostly a flash hider with minimal compensating abilities. I do plan on getting the .300sps suppressor so this was a big reason for picking it. If you aren't going to use a suppressor you can probably find something else that is cheaper/shorter.