I think the upper playfield is great - much preferred over the pro.
- IMO the issue with the Premium/LE is the left orbit MUST be fixed, as the default setup ruins the flow - once this is done, it's great
- The upper playfield doesn't hurt flow at all IMO
- Hoping for higher scoring for Castle Multiball in a code update though

Then you really won't like ACNC, as both/all ramps feed the upper playfield.

On GoT the right ramp always feeds the upper playfield, but it's designed so if you don't flip the ball returns to the left flipper almost as fast as if it were just a ramp. It's your choice - sometimes there are reasons to use the upper playfield, sometimes it's better to just let it return to the flipper. It takes maybe 1 second longer than on a Pro if you choose to not flip. It's not a flow killer at all IMO.

I think some people feel like they have to flip if the ball is by a flipper.

Most Prem/LEs have some level of this issue from the factory.
I was looking into producing an updated diverter, but it turned out to not be cost effective. The reaction from pinside was that the price was going to be too high. I think I had estimated $90.

Like Sulakd posted, the no cost fix is to enlarge the holes on the diverter bracket. With enough tweaking, this method works very well.

I don't think I will revisit making them - the thing is the other fix is simply increasing the size of the holes in the diverter bracket. You aren't drilling anything under the playfield. You take the bracket off, and simply enlarge the holes. After it's installed back in the game, you can't even tell anything was done - other than it working better LOL.