Would ya believe it, 500 miles later that lack of power above half throttle returned.
Carb off again, checked diaphragm again with me bionic eyes, no splits or pin holes to be seen. Then I thought, I'll change it anyway, that's when I found it was not a good tight fit on top of the slide. Fitted the new one & it's brilliant, good for 75mph now with this 200lb + kit, fat ol' fart perched upright. Soaring uphill at 50+ mph, even overtaking cars. I bought a full carb repair kit via AliExpress for £18 inc shipping. Jobs a good'n eh. Yamaha sell the diaphragm/slide as an assembly for £130.

Woohoo! Even the smallest leaks can prevent it from working like it should.

Paul - Brilliant! Glad you found the problem and shared the solution.

A great feeling when we sort something that been giving us grief.

I did a search on best way to fit the new diaphragm, different slide but similar fitment.
This is the method I used. Hopefully might help anybody else who has same problem. Ensure it's fully seated.

https://youtu.be/BQ9ziq5HZb4

Yes it is. Especially when what's busted is between you and enjoyment. Thanks for posting and providing reference material. Many hands make light work. Ride safely.