The Rainbow / Pentair 320 doesn't have the flow restriction problem with Hayward unit does, and usually, the flow control valve works much better.

You can NOT bore out the hole, because the pressure drop across the restriction is what 'drives' the unit.

Be careful about using any water pressure device to clear the lines. Most pool installers are NOT plumbers, and are accustomed to maximum pressures of 50 psi; typically even when working with S40 PVC pipe, which can hold 100+ psi, they do not use joining methods that are up-to-spec. As a result, trying to clear the lines like that can blow joints loose, or even shatter pipes - causing you to go from poor flow, to no flow.