"How Deep is Your Love"?! Or all kidding aside, how deep are your pockets? LOL
The question of diminishing returns is actually very hard to place a finger on IMO. 1000 people could give you a 1000 different answers.
I'd say likely, a $12K Cartridge and a $20K Table, and all other downline components being exquisite, all things combined being right on will probably extract just about all that could ever be expected to be retrieved from any vinyl groove.
But, some might still say, "meh", and would probably be correct to say no matter what and how much spent, it still can never equal a live performance. It will always be a relative shadow and glimpse of. And that can be of any genre, be it hard rock, clasical, jazz, R&B, etc.
Like when I seen ELP many years ago, when they came out with their Brain Salad Surgery Album. Made my system sound like a mere $9 GE hand held Pocket Transistor Radio from the mid 60's. LOL Same with Zep, Jethro Tull, Dave Brubeck live, Buddy Rich etc etc.
All BS aside, I'd say you must be thinking your existing Empire is due for re-tip with a new Stylus on board. That would likely be the simplest-easiest, and least expensive option to keep you playing for the foreseeable future.
Otherwise, moving up costs some better money. And when I'm talking better money, it staggers me at times to see what meager offerings today cost! Something like a Shure M-97, etc., make you go round and round, and say damn, do I want to spend that much?, and then will it really be an improvement which I can justify the cost of? Guess there's one way to find out in that instance.
Depends all where you want to go? How far you'd like to go up the ladder, and how much to spend?
The point of your actual diminishing returns could probably be, "Does it make any logical sense to throw a $400-$500 MM Cartridge on your Table? Will your table be able to compliment and extract the performance of such a cartridge? That's hard to say?
Will it play nice, and likely be a good improvement over your existing cartridge? Very likely it will. And might even perform better if you one day transplant that cartridge to another, better table-arm.
I've heard some crazy stories like this, about guys who fish, that's their entire life. They've had a few boats prior, but now decide to go out and buy a $50K Ranger Bass Boat, And they're pulling it with a $700 1978 Silverado Pickup Truck. That's what they wanted, and that's what they got.
They don't care that they haven't changed their underwear or socks in a month, what they care about is that boat! LOL