I have had two sessions now on my fanatic stubby FSW105 using the thruster fin set up that came with it, 12/20/12cm fins. Vs a 34cm version of the fin in the photo - maui ultra fin 34cm x-ride.

The difference as expected:
Thruster was looser and turned tighter.
Single fin got planning earlier.

The wind was 16-18 knots. I was surprised that the early planning difference was only marginally better on the 34cm. About 2knots of wind.

Also the top speed was only a couple knots difference.

Also the 34 jibed just as well.

I was expecting a massive transformation in the boards performance but it was only marginal.

Conclusion. I'm going to try a bigger fin.

I've got a 105L tabou 3s fsw and I was out on Friday doing fin tests in marginal wind with my 6.2 Blade wave sail. I was testing a 27 ,28,32,36,38cm fins. It's always a trade off in early planning vs manuverabilty. I should have been out on my 6.9 cross however just sold it.

My board can comfortably handle both the 36 and 38 with the 6.2 and there is a improvement in early planning over the the other fin sizes however there is a loss in manuverabilty. The 38 is a choco spirit which has a thicker and more forward foil which gives it very good bottom end at the cost of some Top end. The 36 is a vector freeride which has slightly better Top end at the cost of some bottom end. There is a type as well as a size effect. The 38 for me was getting a little hard on the ankles once I got powered Up hence I prefered the 36

I'm not sure how the fsw stubby goes, however I would concure with you that you need to try a larger fin a 36 or 38 , however the type of fin is also important

Shifu said...
What size sail?
A couple of knots is a big difference actually, both for earlier planing and top speed. About what I would expect.

I was using a 5.3 blade pro

I think FSWs are 18+ knots boards, whatever their size. So a 105L FSW is not an early-planing board for a lightweight or middleweight, but an 18+ knots board for an 90-95 kg guy or maybe an inexperienced low-intermediate middleweight. If you want to get planing early with a 5.3, at your 75 kg weight you should get a 100L freestyle board.

EDIT:
I think this is well worth reading.

http://www.loftsails.com/tips-for-building-a-windsurfing-sail-quiver.htm