My first road tubeless were 25s, some top of the line Gavias. Great tires that rode wonderfully, though my frame of reference at the time was 23/25mm tubed tires.
Then I tried 30mm tires (Schwalbe G-One Speed) - suuuuper awesome for a bigger guy like me (~200lb at the time). Being able to run them at 70psi was a game-changer - long rides were much less fatiguing and they rolled really, really well; a magic carpet ride on so-so roads. I'd been so happy with them that I stuck with them exclusively for a few years and told myself that I wouldn't buy another bike unless the frame could clear them.
This year, I tried some 28mm Hutchison Fusion 5 Performance 11-Storm. My weight is down, so I've been able to run them at the same ~70psi, too. Not quite as cush as the G-One Speeds, even at the same pressure. They seem to roll about the same, though they feel notably more nimble (I don't know if it's a slight change in trail with the slightly smaller diameter or the lower weight, but it's certainly there).
On spirited group rides, neither has been a hindrance, neither in a straight line nor when thrashing out of the saddle (in a pinch, I have taken some 35/38mm tires out with the group and they were a *lot* more work). Solo, they're both definitely beneficial (faster) over longer rides (3+ hours) just because I'm fresher and less beat up. On shorter rides, I'm doubtful that any penalty (aero, weight, etc) would be statistically significant.
No desire to go back to 25s.