David,

I would hold off making any hole size changes right away. You'll have 100 MPH air coming into that chamber, where it will slow down and pull out some heat before being sucked out the back. You may be surprised at how little it actually takes.

On my one pylon racer I have a hole 3/16" dia inlet and 1/2" dia outlet at the tail and it does a great job.

As for spinners, try the DuBro #260 series. The bolt-on construction is much better than those CG crunch on types.

Andy