indeed

Personally I always thought nice bearings (like DA or the Hawk ones Brian listed) + an aero cover like Berner used to make would be ideal. But no data to back that up

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My Blog - http://leegoocrap.blogspot.com

But that's just it. "May be". "Half a watt".

It's not a small thing for a large manufacturer like Shimano to retool mass-produced parts. When all that retooling might buy them is 0.25 - 0.5 watt differences that we are not even sure exist, is it any wonder they don't bother?

Hell, they could get far bigger differences by improving their not-very-aero "aero" wheels. There's some relatively low-hanging fruit right there. Yet they don't do that either

Because most people don't use wax lube?

I think two factors not to neglect in answering your question:

1. Shifting performance. Introducing a longer, more flexible cage can apparently make shifting more vague. Ok for a triathlon, not so good for road racing.

2. General reliability. Chain tension decreases, so you may be more liable to throw a chain during a race. Then, I think of my friend, whose oversize jockey wheel and pulley (IIRC a Ceramicspeed) disintegrated when he was doing the junior Paris Roubaix.

Anyway, compared to the lack of implementation of oversized jockey wheels by Shimano, more astonishing to me is SRAM having forced smaller chainrings and a 10 tooth cassette on consumers and racers!

I agree. The watt/$ benefit just isn't there for the expensive ceramicspeed oversized pulley.

having said that... I broke my Dura Ace carbon pulley and had to replace it with something. I ended up buying one of these oversized carbon pulleys from Aliexpress ( 'WAL' brand - they range from $50-$80; i went for the $50 option) and the jockey wheels are impressively friction free (spins 'forever') and the carbon pulleys are very stiff (if not overbuilt). I have no clue if it translates into any meaningful difference. 1 watt seems so trivial when I'm likely leaving several watts on the table from training/diet/fatigue/etc. But for my circumstances (broken pulley), it was a good buy.