For those that have/use them. what is your experience using them compared to a 'stock' fin like MFC ?

do you find them too flexible ...or ? I have a few sessions on a thruster set. Jury is still out. I had one good session....and the other were inconclusive (wind was all over the place ). the next time out I went back to stock MFC and that was a decent time .. just trying to figure out what to expect...if anything. my fin sizing is similar to the MFC setup. (I miss my black project setup....I can't find a thruster pair mini Tuttle )

I like a G10 center and K4 thrusters

I'm relatively lightweight myself and don't use anything above 20cm, but I've never had any problems with them being too soft. I like the feeling of them flexing a little bit, the G10 can feel a bit "dead" in comparison. I use them for both thruster and quad set-up, and I like them a lot. The stiffness depends on the model (profile) and some of their fins are also offered in different plastics.

I mainly have the 'stubby' model....front & rear.

also I read somewhere that it is better to change the side fins (up or down) rather than changing the center fin...for stronger or lighter wind ? I always changed the center fin.

I really like them and I'm not light. I find the Stubby very easy to ride as a quad rear combined with the 2-3 degree Ezzy fronts.

I like the incinerators, with the AI version in the front.

My spot has a lot of on-shore and side-on days with current going with the wind whenever we have decent waves. And the waves, when there are waves, are mushy and often close together (not coming in sets). So, all that makes for pretty difficult conditions.
I have a 99 liter Fanatic Stubby TE and it was my first multi-fin board. The side and center fins can all be adjusted, as opposed to FSW boards that have fixed fin positions. So, it took me some time to figure out the right fin set up given our conditions and the variations in fin placement on that board. The stock fin felt way too small, so early on I tried the K4 Scorcher (stiffer plastic than the K4 Stubby) with the Scorcher sides in a 22cm and 10cm for bump and jump with a larger sail (6.0m). Does the trick if you're trying to avoid buying a Freewave board, I guess, but I probably had too much distance between the side and center fins, so was overcompensating. Haven't bothered using the 22cm again, but could see how a 20cm Scorcher might be okay. Seems to be a similar set up as what's used on the Goya Nitro in a similar size.

I also tried a 20cm K4 Stubby with the 12cm Choco side fins that came with the Fanatic Stubby TE (mostly 5.7m and 5.2m sails, comfortably powered up to overpowered). Was ok, but I think the plastic used for that model has too much flex when our conditions are at their meanest (almost directly onshore, gusty, generally unpleasant but still better than work). I also tried the stock center fin again, paying more attention to fin spacing, position, etc, and it was ok, but would not be a great choice for me for most of our usual conditions.

So, because I'm cheap and had an extra MFC G10 single fin from 20+ years ago that I don't use anymore, I recently took the outline of the 20cm K4 Stubby and traced it on the G10 fin and sanded it down to match the K4 template. I'm using that with the stock Stubby TE side fins and it feels much better, so far. Faster, less spin out, no problem with maneuverability. If I was to break out my wallet for fins for that board again, I'd get something similar to the MFC VF thruster set.

FWIW, I have a Goya C4 with the stock MFC fins. Love it. Not changing anything.

You might try the 20cm Leon in stiff. I've only used them as a quad rear but they are my favourite for onshore.

In UK, local wave beach most wave sailors change out to K4. Scorchers were the common ones.