It is not just a question of fin brand, type as it has to match your kit and conditions.
Recently I have been trying some fins that I had been really happy with in a different board / sail combo and can't seem to get them working well.
It is not the fins but they are not working with the set up.
I can only comment on fins that I have used.
Biggest fin I have done 40 with is a 32 Tectonics Gold Wing (iso 90) so generally smaller 28 down depending on your weight / sail size. I am 75kg and all of my sessions 40+ have been with 6.3 or 5.6 Loft Blades and 5.7 North Warp early on.
Flat water in strong wind is the best chance of 40+ so weedy sites are good. Delta fins have been used in really weedy stuff here to go 40+
I have used MXR speed weed 22cm (M SL 91) (these fins need you to be really powered up). I have also used the MXR UFO 30 10% 22 (50 wide speed board) to go 40+
I haven't tried the new Tribal / Lockwood fins (lots have and rave about them) but my best 5x and a few 40+ sessions have been on a KA /Lockwood 23 (50 wide speed board)(very versatile fin)
Tectonics Falcon 26cm (50 wide speed board)is a really nice fin (but needs the right board to match)
If you look at GPSTC or gps-speedsurfing you will see there are many fins that have done 40+ in the right hands.
You need enough lift to get the board to fly - too small and the board will have too much drag and you will crab sideways cross wind
Small enough to reduce fin drag and keep control.
Generally a raked fin will give better control at speed.
So...choose a fin, find your spot, wait for wind....good luck and have fun experimenting.
PS. If you are a light weight try using a weight jacket (neutrally buoyant) as it will give you a much better feeling of control in big gusts and help your speed.
Love the MXR weed speeds
As Lao said tho they really need to be powered up - ie 25+ knots wind to get the JP 59 to float.
I bought a bunch of new fins, have a 26cm Tectonics fury to try out, but could be a while before I get the wind to test it out on!
Good list of fin websites on this old slalom thread
www.seabreeze.com.au/forums/Windsurfing/General/Whats-your-fin-of-choice-for-slalom-speed/
BLACK DIAMOND FINS : www.blackdiamondfins.at/en/ BLACK PROJECT FINS : www.blackprojectfins.com/ C3 FINS : www.c3-fins.fr/ CARPENTER FINS : carpenterfins.com/products.html CHOCO FINS : www.chocofins.info/ ELK FINS : elkplus.lv/fins1/news/ ERD FINS : www.erdproducts.com/ F-HOT : www.f-hot.com/ GASOIL FINS : ailerongasoil.free.fr/ HURRICAN FINS : www.hurricanefins.de/ I FINS www.ifins.eu/ KASHY FINS : www.kashyfins.com/ KING FINS : www.king-fins.com/index.html KW FINS : kwfins.wordpress.com/ LSD FINS : www.lsdfins.com/ MAUI ULTRA FINS : www.mauiultrafins.fr/fr/ MAKANI FINS : www.makanifins.com/collections/windsurfing-fins MB FINS : www.mb-fins.com/f/home.asp MXR FINS : www.mxrwindsurfing.com/finoverview/ SELECT FINS : www.select-hydrofoils.com/fr/ SMART FINS : www.smartfins.com/technologie.html SONNTAG : www.sonntag-fins.com/ TECHART FINS : www.techart.it/ TECTO FINS : www.tectonicsmaui.com/ THE BOSS FINS : www.thebosscustom.com/?page_id=186 TRIBAL FINS : www.tribalwindsurfing.com/ WINHELLER PROTOTYPES : new.winheller-prototypes.com/?page_id=640 Z FINS : zfins.eu/
Mean line meanline.com/
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From 349euro/ $A500 a pop.
They will need to be good at that price.
I'll tell you when I get 40 although you might be old and grey by then. Well I know I will be!
Any fin designed by Chris Lockwood would be up there too.
Select to expand quotelao shi said..
It is not just a question of fin brand, type as it has to match your kit and conditions.
That right there sums it all up perfectly! 
But, unless you are a giant in stature and are forced to sail in less than 30 knots of wind, you should only be considering fins smaller than 25cm depth, and if raked, 23cm, preferably smaller. At 40 knots these fins will have more than enough lift for any size person, and way less drag. And if that fin does not feel right, check your board or sail choice. 
I found a good article on fins, focusing on formula but broadly applicable to all high performance blades.
carbonsugar.com/design/everything-you-should-know-about-fw-fins/
