Ok. I started this thread with the question as to 'whether expensive carbon fins would make any worthwhile improvement to maximum speed.'
Lots of input, ideas and thoughts... and lots of sailors have contacted me personally to share insights. Thanks to all!
My conclusion... there are many brands of fins that produce top quality fins at a variety of prices. IFins are great, though expensive; Zulu and Erd (had never heard of them) have loyal followers; local brands, Tribal and Atomic are getting fabulous results, have many happy users and can be had at a reasonable price.
For myself, I do not have the finely tuned skill-set to really tell the difference between most high quality fins, so will support 'local' and when in the market for some new blades, will go Atomic and Tribal.
Cheers.

Second best after testing yourself, is reading extensive test reports from others. And then focus on the charateristics of the fin, how they are different in their behavior. Lift does not equal lift, because you can have nose lift (tailwalking when you take it to the extreme) and tail lift, where the fin lifts the board as a whole.

There's some excellent reports on french W33 forum. Just paste the URL into google translate, and it becomes understandable even when you don't read french.
They have a specific subforum for tests on materialwww.windsurfing33.com/forum/viewtopic.php?f=15
For instance this one, 6 pages on ALF custom fins, which is quality and performance wise, comparable to chopper
www.windsurfing33.com/forum/viewtopic.php?f=15&t=115094
And this one about Chopperwww.windsurfing33.com/forum/viewtopic.php?f=15&t=121613
And another one comparing 7 semi-customswww.windsurfing33.com/forum/viewtopic.php?f=15&t=118316

On the dutch forum, also a comparison of several semi-customs (King, Chopper, Sonntag, CA)forum.windsurfing.nl/viewtopic.php?f=62&t=13903708

Tribal carbon MK2, apparently very very good trust-giving fin
forum.windsurfing.nl/viewtopic.php?f=62&t=13902338

And finally, our dutch pride, reports on Finish fins
forum.windsurfing.nl/viewtopic.php?f=57&t=13898076

My take: Finish, Sonntag and ALF give the best price-performance-quality relation.

I have a few Lockwood Carbon fins. 38 a my favourite while the 29 is super slippery but I've not had a good windy day to really get a feel. 38holds on really well and corrects from spin out effortlessly and I think that I can feel it flex and stuff. Very fast too. In the $450 bracket and I'll be getting more. They do appear sometimes on the B&S.