I don't make it a habit to open links, so you're on your own there.
Increased distance and sufficient capacity? It depends on what type of fishing you're planning to do with your dream reel. One reel doesn't fit all applications.
The bearings in the Ultra Cast UC6500C are outside the spool, not inside. Many of the other ABU reels I own, in various sizes, which do not have the words "ultra cast" in the nomenclature, have the bearings on the inside of the spool. Like I previously said, there are different variations of reels called by the name "ultra cast" that ABU manufactured. So what I contributed above is all I can help you with.
You said you were thinking 7500 or 8000. I'll say it again, whiffling is marketed for quick bursts at short distances, not suited to 7500 or 8000 reels. If you want to cast a 7500 or 8000 amongst some lily pads all day long, then by all means drill some holes to get the reduced inertia you're looking for.
"Do you think casters at an event would be willing to "give it a go" between roundss or could it throw their timing off and hurt them in competition, later on ?" Good luck with that. During the competitions I attended, there was no such thing as "between rounds". You ever attend one of those meets, lately? Maybe they've changed, but knowing how much they need to get done, I doubt it. Even if there is a break, for whatever reason, they've got other things to concentrate on.
I'm still interested in the response you get from your UK friend. Let's hear it when that comes around.