Is your transbrake valve body for the Pro-tree or is it for the Full-tree? The pro-tree usually has two dumps for releasing reverse, so it is applying more fluid to counteract the amount that is being bled off and the passage would probably need to be a little larger. A usual sign of the pro-tree brake is that you have to apply the transbrake button to back up. If it is the full tree brake, it doesn't try to release as quickly and therefore doesn't have as much volume of fluid applying reverse. That would be the determining factor of whether I drilled additional holes.
It doesn't sound like your transmission guy has the expertise with this racing glide, so you should probably get a few of the books and start again with a different set of eyes.
One thing to keep in mind, is that when you increase the oil feed hole diameter, you can sometimes decrease the pressure but increase the volume.
Good luck